It is not so that it rains a lot there. Misinformation.
Tall islands can get a lot of rain in some years and very little in another. And this may vary too from one place to another. The Pacific is not your local lake - look at the chart, learn to read the scale. Walk places rather than drive everywhere - you will get some understanding of the DISTANCES then, perhaps.
Get the biggest
tanks you can. Then some jerries too. You may end up where it is not that you will get water - you will get ASKED for some ..., perhaps.
Perhaps not.
Expect little water in the low islands. Tuamotus and the likes. Unless they have a watermaker. Very many do not.
Or else you may get water but it may be of dubious quality (read about amoebas some - this may help you decide on what you want).
Water up on the continent. You will get water in
Hawaii and the Societies. And then it is all up to where you go. Ask about specific places. Distances are huge, water situation varies a lot.
Get a watermaker if you afford one. That's sure.
Catch your own, too. Rain.
Good luck.
b.