I'll be diving at Lembeh in a couple of weeks, and -- assuming I actually receive the custom focus ring I've been promised by Harald of Seacam before then -- I intend to try a new lens combo:
Sigma 150mm macro with Kenko 2X teleconverter, for an effective focal length of 300mm! 
I've tried the combo out on land, and it seems to work well (manual focus only), but focusing will be a challenge at 1:1 range. I don't expect it to be very effective if/when there's much current, but it should work on some of the "calm" dives at Lembeh.
BTW, this lens/TC combo fits inside my Seacam macro port with 50mm + 35mm extension rings combined. In order to add the 500D diopter, an additional ext. ring is needed, so I doubt I'll do that much (I'll just use a wet diopter if I want more magnification).
Anyway, wish me luck, and any helpful hints/tips on using this lens/TC combo would be greatly appreciated!
