When you say "TTL" do you really mean TTL or just an electrical connection (like Nikonos) or do you mean electrical connection that also supports TTL with your existing strobes?
Most people find they are able to shoot manual strobe mode quite effectively with just a little practice. The fiber optic strobe connections are generally 'safer' than electrical connections with regards to flooding/corrosion/loose connections.
My PM1/EPT-06 with dual ULCS 8" arms, YS-110a strobes and 4 jumbo stix floats (I swap arms/strobes between Nauticam and Olympus housings) is slightly positive UW. Without the stix floats it is probably less than 1 lb negative. I would not worry about float arms until you figure out all the doodads and thingamajigs you may add to your system.