Water Collection
1) Make more bottles. I usually have at least 4 one in my cooking pot with clean water one on me with clean water, one on me empty to collect dirty water when I visit POIs and one for oils.
2) Collect water at POIs. Look for birdbath/water collectors. At the beginning of the game in the starting POI there are 5 birdbaths, 2 cans of water and a free water condenser membrane that is 22 water. That should last you about 6-7 game days right there in itself.
3) The greenhouse POI has a TON of water in the main area I mean I took 30 from it once and it never went down.
4) Upgrade to large bottles as soon as possible. This is so huge and helps so much.
5) Always have dirty water processing in your water purifier. I mean every-time I come back to my ship I’m either putting in dirty water, taking out clean or doing both.
6) Make membranes and/or collectors. One water condenser and 2 rain collectors should be all you ever need. It rains a lot in the game and making a membrane only cost 10 synthetics.
Food Collection
1) Park your ship so you can always send down your lure. I park my ship on the edge EVERY-TIME I land at a POI and send down my lure when I leave the ship and then I have something there waiting when I get back.
2) Always have something on your lure AND in your pot. Things that are in your pot cooked and reeled up on the lure do not spoil. The timer does not start ticking till you pick them up. So, I always leave something in each of those spots and then just take them as needed.
3) Unique lures. USE THEM everything you need to make unique lures are at the green house. Now early on I understand you might not want to use your polymer on this but you can make them (after you research them). 100 synthetics and 5 oil = 5 polymer and then all you need are those yellow flowers 3 of them and you can make a lure with 10 casts. I usually make 2 when I get to my first greenhouse and that lasts me about 6-7 days.
4) Make a crossbow and kill the crickets they are easy, also at every greenhouse and will get ya like what? 50 food.
5) Fruit/veggies. I personally never grab these unless I need them for research but they are easy to collect and then cook and are usually in a relatively high abundance. I think the lure is easier but this is also a good alternative. If they fix/make the freezer an actual freezer this would probably actually be the best method of keeping food on hand but for me things spoil too quickly currently.
Final thoughts is that you can also expand on these.. have 2 cooking pots, have 2 lures down at a time, make 2 water purifiers, have 10 water collectors.
Food and water demand in this game is a walk in a park comparing to Raft. I always chuckle when someone bring up this topic.
By the way, pool of dirty water at Greenhouse tower contains exactly 1000 units and can be drained completely.