1.
The dead men of Dunharrow - they're dead anyways, so they won't die by drowning.
2.
Nazgul - Not really alive either.
3.
Sauron - Well, he's been through a Sea disaster once. It won't hurt him if he goes in it again.
4.
Smaug - I'll let him fly after the boat if he'll stop getting mad at people for accidentally taking his cup.
5.
Legolas - he can walk on snow, so maybe he'll walk on water.
6.
Thranduil - to keep Legolas company.
7.
Balrog - if he won't be able to fly after the boat, it's prove that he doesn't have wings. Just an experiment, you know.
8.
Gandalf - what if he starts uncloaking on the boat?
9.
Radagast - his eagles will save him.
10.
Galadriel - with Nenya she'll be able to command the water to keep her afloat.
See, by throwing these out, we don't actually lose anything.