I like Jim's choice John. Especially if your shooting a Zen glass dome with coatings that seem to do a better job controlling flare in shallow water situations like you'll be in with the whales. The other side of the coin is that the large dome provides for a nice negative bouyancy so you don't have to worry about dropping your rig into the depths. When I was in the water with the Sperm whales I ended up using both.
Be sure and let us know how it goes in Tonga, That trip is high on my list.