Floatation, one of the all importants and generally if it's under the floor then one has to sit on the bottom of the hull in times of dire circumstances.
However in a small tinnie not generally a lot of space to put it where it should be (around the top of the gunwales) and if it's under the cross seats then I wouldn't modify them in any way (as they generally an integral part of the hull design) or the floatation under/in them.
Cheers, Kerry.