There should be dozens of them, but I'm stumped here. Nothing other than "tub" and "frying pan" comes to mind. If someone witty wished to insult your boat, yacht, or ship, what would he or she call it?
Calling a ship a boat, is sometimes considered derisive. Also, calling a type of boat by the name of another type of boat is sometimes considered insulting by its owner. If the boat is a man's mid-life crisis, adding "little" to whatever you call it will surely sting a little extra.
Some ideas & examples:
Little boat: Aww, I like your little boat.
Dinghy: Your dinghy is so cute.
Driftwood: Is that a boat, or a piece of driftwood?
Tub: If your tub is here, then how do you take a bath at home?
Rust bucket: Good thing I had my tetanus shot before climbing into this rust bucket.
an old or clumsy boat; hulk; tub.
On the UK canal system, people with traditional narrow boats (70 feet long, with solid steel hulls originally designed to carry 20 tons or more of cargo) sometimes refer to fiberglass hulled craft as "Noddy boats." See https://en.wikipedia.org/wiki/Noddy_(character) for the meaning of Noddy.
Canal and river boat owners who don't keep their boats looking smart are referred to as "Rodney" from the name of a generally incompetent character in a UK sitcom: https://en.wikipedia.org/wiki/Rodney_Trotter.
Traditional narrow boats were often decorated with stylized paintings of roses and castles on the cabin sides. A boat with badly painted roses may be called a "cabbage".
An incompetent canal-boat handler is often called a "sailor" - this may date from the end of WWI, when the commercial narrow boat companies hired unemployed merchant seamen, which was disapproved of by the long-standing canal boatmen, who in were often from families that had been boatmen for many generation, and in some cases literally were born, lived and worked all their lives, and died on their boat.
A modern canal boat that is a poor imitation of the older designs is sometimes called a "Washer josher," because instead of the original hull construction of steel plates riveted together with washers, imitation washers are simply glued onto a fiberglass hull.
A leaky canal boat is called a "watercress boat," since watercress is grown commercially in beds irrigated with shallow running water.
...As has been hinted at by other answers and in the comments, the best insults are usually based heavily on context.
If you can think of an insult to call the boat that isn't necessarily nautical, then you can add floating in front of it, like floating shed, floating scrap or floating turd (or just floater if you like).
One method of insulting the type of boat is (as has already been mentioned in many other answers) to call it the name of another type of boat that it isn't. Often calling it by the name of a smaller type of boat (e.g. rowing boat), or a cheaper type of boat (e.g. dinghy), or a less glamorous type of boat (e.g. trawler), as many other answers have suggested. Another option is to call it sarcastically by a much bigger/more expensive/more glamorous type of boat (e.g. calling a rusty old trawler a luxury yacht). The context will really influence which type of boat is most appropriate.
Any complimentary epithet about the boat said sarcastically (although this would only work if spoken/incredibly obvious it isn't actually a compliment).
An insult could be related to the apparent imminent sinkability of the boat, e.g. sinker or shipwreck waiting to happen; or to the fact that it leaks, e.g. (to borrow from Harry Potter) leaky cauldron.
Other insults could be related to the person that bought/owns the boat. For example, people going through a midlife crisis often buy expensive/unnecessary items like sports cars, but also boats. You could therefore call it a (floating) midlife crisis. Or, if a man has bought a boat and it's seen as a way to "prove his power/masculinity", you could call it a floating penis extension.
Other methods mentioned in several other answers are:
One of my colleagues, who had a sailboat, called all boats driven by an engine, stink-pots. Merriam-Webster
A boat equipped with a motor.
See Sailing Yachts v Stinkpots in the Financial Times. This article, which the FT does not want me to excerpt, well conveys the disdain that sailors feel for "motorized pleasure craft". In the case of the authors of this article, they are somewhat abashed at their disdain.
The article refers to the book Coot Club by Arthur Ransome,
The most entertaining piece of anti-stinkpot propaganda ever written.
I am sure the owners of "motorized pleasure craft" have equally derogatory terms for sailboats, and perhaps another person will supply one.
Mr.A : so how do you like my new luxury yacht!
Mr.B : it's very nice... So what do you call this dinghy?
I don't see "raft" mentioned yet, which would be a fairly disparaging way to refer to a "real" boat or ship.
Definition of raft
1 a : a collection of logs or timber fastened together for conveyance by water
1 b : a flat structure for support or transportation on water
2 : a floating cohesive mass
3 : an aggregation of animals (such as waterfowl) resting on the water
Users of the British canal system usually travel in steel narrow boats. They will use the term Noddy Boat to refer to anything made of fibreglass. the term is purely derogatory.
The term derives from the Enid Blyton children's character Noddy, who uses a boat in several of the stories.
Depending on the type of boat, calling it a tramp steamer might hit the right nerve.
(Also: I used to delight in referring to an acquaintance's father, who was by all accounts a respected pilot of large cargo ships, as a tugboat captain. Made me laugh, anyway.)
This is all based on context. Here are mine.
A "canoe". It's a boat, but it's also ancient technology. In poker a small full house is commonly called a canoe.
How about some imagery, "a floating piece of trash" for instance, or "sea turtle" for a row boat, or simply "row boat". Or a "sea barge".
Better still, "titanic".
Oh, got one. Call it "an anchor".
And "rubber ducky" because chances are the guy who bought it likes it for the same reasons. Perhaps "floating toilet", or "fire trap"
Adjectives can be used as well, such as: worthless, expensive, hazardous, etc.
"life boat" is funny.
Word forms: plural coracles
In former times, a coracle was a simple round rowing boat made of woven sticks covered with animal skins.
From Treasure Island by Robert Louis Stevenson:
I dropped into the hollow, lifted the side of the tent, and there was Ben Gunn’s boat—home-made if ever anything was home-made; a rude, lop-sided framework of tough wood, and stretched upon that a covering of goat-skin, with the hair inside...I had not then seen a coracle, such as the ancient Britons made, but I have seen one since, and I can give you no fairer idea of Ben Gunn’s boat than by saying it was like the first and the worst coracle ever made by man.
Since a coracle is a minimalist human endeavour to conquer seas, it would not be high praise to call any modern day vessel a coracle.
source: COBUILD Advanced English Dictionary
A hulk ie; "a non-functional, but floating ship"
If the vessel is suffering from a lack of basic hull maintenance, it is often called "a floating reef".
Flotsam - the wreckage of a ship or its cargo found floating on or washed up by the sea.