Sharky

free for all:
grab a bunch of pool noodles and hit each other untill you get bored, doesnt last long but is fun while it lasts and you can also fill the noodles with water and blow into them to squirt water at each other

king of the hill: (solos/team deathmatch depending on the boat) (my personal favourite!) :)
if you have a boat pool toy you could do king of the hill as individuals if its a small boat or teams if the boat is a little larger. its pretty simple but its fun pushing each other off the boat and trying to stay on the boat without getting pushed off yourself.
you can decide to make the time to stay on the boat to win shorter or longer if its too easy or too hard.

capture the flag:
capture the flag is so much fun on land but in a pool im sure it would also be kinda fun albeit not as good (ive never actually tried this i just made this up now while writing the post)

if you have a pool divider thing you can put it to split the pool in halves but in the likely situation that you dont all you need is to just get two pool objects (ideally small) and hide them at the end of your side of the pool then you swim to the enemy side of the pool,get the item and swim back to your side of the pool without getting tagged to win the game (you could decide whether throwing to a teammate is allowed)

if enemies are on your side of the pool you could tag them to send them back to their side of the pool or make a jail system where someone else on their team would be elected to be the only person who can get them back out but thats probably too complicated

if you tag the person with the flag (or whatever item you chose as the flag) they just drop the flag where it is and swim back to the verry end of the pool

anyway as you can see theres lots of shit to do with pool toys and i really overthought this a lot i just really like games like capture the flag and king of the hill both in videogames and real life and i should play either of them again sometime