There are some pretty good places in Eastern Europe for punting but Riga is definitely not one of them. Sorry but I have visited lots of towns and cities across Poland, Ukraine, Hungary, Czech etc. but Riga was my least favourite. British are not particularly liked, a lot of the locals (and the Russians) are complete meat-heads and built like brick shit-houses..... but the girls are stunning I can't deny.
If you do punt, try to talk to a local taxi driver for recommendations as they get a kick back from the establishment. You will probably end up in the back of beyond so keep in a group. You should be ok if you keep savvy. If you get into a proper brothel you will find they probably won't charge you to get in but the drinks will cost a fortune so ask prices up front (vodka is often the cheapest option). Try and get the taxi driver to hang around which they often will do anyway to pick up your return fare. You will very likely find all the girls will be Romanian or possibly Russian but unlikely local. You may get lucky and have a good punt, it's really hit and miss but if you are just after a bit of fun on a boys trip then you will probably be happy enough. Expect to pay 80 - 120 quid depending on where you go and what they decide to charge you.
I haven't been there for some years so it may have changed a bit (hopefully for the better).
Have a fun and safe trip.