...Um, this might not be the best time to tell you this, but I'm pretty sure most forgotten beasts don't actually drown.
yeah i realized that when i saw it leave back into the caves and swim off while my dwarves were busy beating each other to death with pants underwater all while the tame aligator population explodes ,due to the lack of dwarves to steal the eggs, to the point where there are so many in multiple rooms that they start attacking whatevers near them, namely dwarves and moar alligator hatchlings
in the end i had about twenty dwarves die to the forgotten beasts flesh rotting syndrome, another fifteen drowned in the great hall and thier rooms, about ten starved or died of thirst, and twenty more died of either berzerking, being mutilated by the zerkers or being chewed up by tame alligator hatchlings
my favourite one of all was this one single dwarf who went melancholy and swam through the fortress to the great hall where so many others had drowned, it was filled up with 6/7 deep water in most of the room, but he was such a good swimmer that he couldnt drown
the worst part about all this is that capybaras were the only thing i wasn't afraid i would lose my fortress to...
but never again, allow me to warn you so as you may not make the mistake i did, if you ever see a capybara with a scar across one eye wearing a monocle and a top hat, with an evil goatee... run away as fast as you can, you've already lost by this point because thats dread pirate capybara the anonymous, he often times employs muscle, and can quite easily plant a mole in your organization, but hes not afraid to get his paws dirty either, not to mention he likes legendary marksdwarves for their blood
and about the whole magma thing, thats my only regret, that i didnt have any to flood my fortress with, and even if i did it would be beneath my mine making it incredibly hard to get at without releasing the beast due to all the strip mining i had done
caveins were not an option either unfortunately, the fb was standing one z level below my great hall, i feel so undwarflike right now having only had access to water