Seven boats does not seem like very many. Why not host a 'bring and buy sale' to raise funds for a few more?
If you don't like that idea how about putting on 'Sub Captain Experience Days'? It would be very popular with the stag parties. Ypu know the sort of thing: 19:00-20:00 - Torpedoing a old fishing trawler, 20:00-21:00 sailing onto sandbanks. I can see myself paying upto 70 quid for that. Would one have to bring one's own white knitted polo neck?
On second thoughts these ones don't even have proper periscopes so what's the fun in that. I'd rather go to Prague to drink, shoot and whore myself to death instead.