Yes it's a good overclock. My 939 isn't stable over 2.6Ghz - mainly because it starts throwing Prime errors anywhere over 49C, and I haven't moved to water yet.
I pushed mine, on air, to 1.62v. For a very short period of time. It didn't die, but neither was it happy with those volts. The point is, as long as you aren't frying it, and at under 30C load you certainly aren't, you can push the voltage higher. People on good water setups like yours have more leeway with the voltage. I have seen Venice chips well over 1.7v, but I wouldn't consider that a safe risk - those people are using phase change and sub zero temps. That is just pushing really, really freakin' hard. Heat will increase exponentially as you add voltage from what I understand, so if you do push higher than 1.5 (and at those temps, you certainly can push harder), DO IT SLOWLY, and watch your temps carefully. With water and your current temps, I wouldn't be afraid to test up to 1.6v at all - but I would still do it very slowly.
If your Venice is like mine, it will have a certain temperature at which it simply quits working properly and will lock up and throw errors (like I said: 49C for mine). That temp is likely well over 45C, so if I had a water rig and was in your position, I'd be slowly adding volts up to a 1.6v - 1.65v max and CLOSELY monitoring load temps so they don't break 45C (that's just me, others will likely give you different advice). I'd stop adding volts when I hit 45C load and/or 1.65v, and be quick-testing with SuperPi for my max Mhz, and then using Prime95 to test real stability - an overnight 10+ hr test when I found what I thought to be my max Mhz. If Prime 95 wouldn't do 10 hrs - I'd back off slowly till it did, and then run as many benches as I could throw at it to make sure it was rock solid stable - OCCT, the 3dMarks, Aquamark, Everest, etc.
Of course, if I got only like 100Mhz out of moving up from 1.5 to 1.6v, I would probably back down to the 1.5v and be happy with the lower power consumption - I would want at LEAST 200Mhz per .5v to be happy; that's a lot of heat. Of course, if 1.65v gets you to 2.9Ghz and you're still under 45C, well, it's up to you if that's worth it or not. Of course, YMMV, and for all we know, you're already at the max Mhz your chip is capable of at any temp. Then again, maybe it will do 3.2Ghz; each chip is different.
Whatever you do, go slow and you'll be fine. Measure your voltage increase against your Mhz increase and decide whether or not it's worth it. Be very, very careful of any voltages over 1.65.