In the phrase "life boat", life in a nominal adjective: a noun acting as a description of the type of boat. In that case, "life" will never be plural.
Two of the life boats sank.
Life alert systems are important in home health care.
The plural of life (lives) will certainly be used in many instances, but always as a noun and not an adjective.
Many lives were saved by other emergency boats.
Alert systems are important for the lives of elderly in home care.
If however you wish to use the possessive of life, something that belongs to life, then use the apostrophe and s as with most possessives. In this way, possessives do not change the structure of the noun involved.
It can be hard to keep up with life's chaotic changes.
All of life relies on each other as though we were the crew of a huge boat called Earth. Let's not capsize life's boat.
If you wish to check if something is a possessive, try to reword it with the word "of":
the chaotic changes of life
the boat of life