Some themed teams can return some surprising results, but nevertheless, you'd rather want a well balanced team with variety. Some themed teams will work better and worse than others - apparently it worked well, in this case.
It also depends on who you're going to subject your team on. A team of baton passers might prove to be useful if your opponent doesn't have a hazing back-up, and if he doesn't, you'll return some decent results. Though you'd be stupid to go without a p|hazer.
Type-based teams are often the worse since you have a high weakness to one or various types. In this case, the water based team has a 3x+ grass and electric weakness. Something you need to be fairly careful of.
In my opinion, theme teams are for fun, they shouldn't be used for competetive battling unless you're very confident and experienced in whichever pokémon you're going to focus your themed team on.