I have FL-50R flash unit, too.
The first doubt is: Fantasea FFL-50 case or real substrobe, like Ikelite DS-51, for example?
Another one: If I decide to take real substrobe, would I buy TTL converter (e.g. http://www.heinrichsweikamp.net/blitz/en/oly-ttl.htm, 174 EUR)+weaker strobe unit, or stronger but manual power regulation unit?
If you have some other solution in your mind, please, be free…

Price limit: $500 + shipping

Thanks in advance

When looking for TTL converters for Olympus, check out Athena, They make numerous specialized products for Olympus http://www.athena-opt.com/english.htm I have used both the heinrichsweikamp and athena. The Athena TLL converter is in the synch cord and works nicely if you buy the correct strobe. Warning, I purchase the Athena cord from a company in Japan online and spoke with someone who barely spoke english. At the time, Sea & Sea 350 Pro was just coming out and it looked to be a very powerful strobe (which it is) and wanted to use it in TTL. The person helping me stated it “should work” well, my fault, It didn’t. I have a Brand New (one aborted dive) Sea and Sea 350 Pro for sale now. Paid $1250 (USA) with extra battery.  An body who shoots manual or has a Sea & Sea housing, make an offer.