Some things that help:
All water forms are immune to drowning. There are some large mobs you can simply stand in front and they do nothing to you and you whittle them down. You can get huge amounts of exp for them, even with the lowest water form, if you have the patience. That means it may take you 5 minutes to kill the mob but you get like 1.5k to 2k. I can solo a water major from cradle to grave without issue just using this concept. And in a reasonable time.

Water areas are generally not populated at all. So it is not like you have tons of people coming to PK you.

Once you hit your 2nd to last water form, depending on the form, you can solo sahuagin scouts and such with little risk. So that means level 30ish you are golden. There are also other areas you can solo things without interruption, like the aboleth lair, coral head, kuo toa lair, triton palace, even swamps, which means you can use your water form to fight wraiths and zombies in the new area beyond High Tower of Sorcery.