Every seed should bring enough water (at least I think so)
According to my experience, it is so. They are "random" (well, not really, but you get the idea, I hope) but they ARE there, in any seed I tried. You may find interesting this post of mines about RW hydro-geography.
Would like to see more water in general too (earth ~70% water, 30% land)
Yes, true. But this is true for the Earth considered as a whole: over millions and millions of kilometres square, you find this ratio.
But if you take on Earth an area as large as the area module of RW (a few kilometres wide, after which it repeats), you may find any ratio, from 100% earth / 0% water to 0% earth / 100% water. And the more water you have on RW, the less earth you'd have to build upon.
Not that I wouldn't like more water bodies, I like them (lakes more than "seas"); simply, anything has to be considered in the whole context of playability and player experience...