If you're shooting from a boat, any 300 is an SOB to handle. If from shore, it's not such an issue.
I'm wondering why you need the 2.8. Limiting DOF is one thing, but the jump in IQ over an f4 version simply isn't worth the price difference. I've got both in the Nikon line, and I only bother with the 2.8 when I need to hold down DOF.
Next question, depending on the magnitude of your current crops, would a 400 make more sense? If you're not chasing your tail on the light issue, a 5.6 won't break the bank.