Should I power up this Venusaur?

It's usable but not ideal. You'll still have the upperhand over Vaperons but you're looking at taking out one rather than two. He's not a great dodger and the attack isn't as strong.

If its rare enough in your area, I think it does deserve some dust even if you just use to cover Rhydon and other weak vs water-types in gyms. I have a Vileplume 2005CP with the same move-sets that I use for that. And on attack, RL/PB isn't that terrible if you're weak on good water counters as you said. Not going to melt them like SB, but PB can work with some good dodging.

You could also use Jolteon's as a water counter...(I do)...Not as nice as a powerful Venusaur, but it gets the job done while you hunt for more candies...

Venusaur is handsdown the best water counter, in my opinion.
It is superior to Jolteon thanks to resistance and more hp.
In my experience it is a good dodger too...he looks fat assed but he jumps well XD
If you don't have easy access to Bulbasaur for evolving more, consider using this one ;)