But what do you call a country that has enough water for drinking, crops, forests, etc?
You should realize you're talking about two separate things:
1. Natural use of rainfall.
Countries (like Israel as opposed to Arabia) or regions (like South China as opposed to the North) are usually described as
Plentifully supplied or moistened with water...
Quarterly Review, Apr. 1919, p. 347:
The stirring population... of well-wooded and well-watered Asturias and Galicia.
Well-watered countries are going to have natural grasslands and forests, although not necessarily rain forests or troubles with monsoons.
This follows the word's original usage, which referred to good-but-not-swampy farmland. There are other terms for 'supplied with rain' or 'water' like 'rainful' or 'hydrated' but they aren't typically used in reference to land.
2. Human use of any sources.
North China, Southern California, and Saudi Arabia are none of them well-watered, but they (thus far) do not generally suffer from insufficient water because humans tap groundwater, syphon distant lakes and divert rivers, use petrol to power massive desalination plants, &c. On current weather patterns, they may not be able to maintain large forests or even abundant grasslands, but they're (thus far) able to ensure sufficient water supplies for their agricultural and residential needs.
This is not a natural state and not a problem under discussion, so there doesn't seem to be any set way to describe it in English.
For example, the UN—which deals abundantly with water as one of its global issues—contrasts "water scarcity" with "access to sufficient water for personal and domestic use";¹ "water crisis" with "an adequate supply of quality water";² "scarcity and misuse of fresh water" and "water scarcity and pollution" with "water and land resources... managed more effectively", "apparent security of existing water resources... and... sustainability of future supplies", and "integrated water-resources management";³ "the growing scarcity of water" with "environmentally sustainable development";⁴ "water stress" and "water shortages and lack of access" with "good access to drinking water and sanitation";⁵ "water stress" and "water risk" with "water security";⁶ &c. It mentions "reasonable access to safe and ample water supplies"⁷ and "availability and sustainable management of water and sanitation for all".⁸
Obviously, there are terms you can use. The best are:
Rich in water; ant. of water-poor.
Ignace Steiner, Elementary Reader, German & English, 1846, Vol. II, p. 281:
Lot... went to Sodom, which water-rich like a garden was.
UN Water, "Asia and the Pacific", 3 September 2014:
Water-rich countries, such as Malaysia, Indonesia, Bhutan and Papua New Guinea, are facing urban water supply and quality constraints.
Possessing an abundant supply of water; ant. of water-scarce.
FAO, "Coping with Water Scarcity", 22 March 2007, p. 6:
In many countries, especially the larger ones, there are both water-scarce and water-abundant areas, such as in Brazil, China, and Mexico.
Betsy Otto, World Resources Institute, "New Study Raises Question: What Don't We Know about Water Scarcity?", 29 May 2013:
Even in places that are historically water-abundant, growth in water demand is outstripping available supply.
Possessing a secure water supply, lacking water insecurity.
UN High Level Panel on Water, "Making Every Drop Count: An Agenda for Water Action", 14 March 2018, p. 26:
¹ Winpenny, J. (2015), Water: Fit to Finance? Catalyzing National Growth through Investment in Water Security, report of the High Level Panel
on Financing Infrastructure for a Water-Secure World.
Global Water Partnership, "The Water Challenge", 15 February 2017:
A water secure world means ending fragmented responsibility for water and integrating water resources management across all sectors—finance, planning, agriculture, energy, tourism, industry, education, and health.
'Water-sufficient' and 'water-sustainable' do show up occasionally (e.g., here and here) but much less often, particularly at the UN & NGOs.
However, go back and look at the UN examples from before. Their main idea (and that of the NGOs and other professionals in this field) is that you shouldn't use these words, except in reference to that distant shore where lambs lie down with lions, e'ery valley rises up, e'ery mountain and hill lays low, e'ery rough land is level, e'ery rugged place is made a plain, and the industrialized world curbs its carbon emissions. Even (especially?) the places that seem sufficient now have regions of insufficiency, have unsustainable water management, and remain insecure in the face of ongoing pollution and climate change. Talking about them as if they were already autarkic is irresponsible because no region on earth should behave as though it were sustainably self-sufficient into the indefinite future.
TL;DR: Go with 'water-rich' to reference the unequal access countries have to the world's water resources; go with 'water-abundant' to reference that some country just has a lot of water; use 'water-secure' to reference that some country does not suffer from water scarcity; but, better still, don't use any of them if you can help it. Current best practice is to focus on major scarcity where it exists and improvements to resource management where it doesn't.