I can dive the Puget Sound in a 7/8 mil wet suit but only for a couple of dives. Most of our diving here is done in the fall, winter and spring so cold temps are the norm. I like my dry suit for diving up here so that I can do more than a couple of dives and stay warm. My wife stays pretty warm as well when we dive. She is wondering if diving a 5 mil will cause her to over heat. As for me, I work outside all year long so cold is not a huge issue and my wife runs pretty warm.
Drift diving is a little different from working outside in the cold. When you're working outside, you're working. The trick with drift diving is to do as little work as possible and let the current take you where it will. Also, you're fully submerged when you're diving. I'm guessing when you work outside, you're not naked.
Also, in April, I doubt you'll see water temps over 80. Those happen in the summer and fall, but spring is the tail end of the winter cooling and you'll most likely experience upper 70s.
Finally, it's hard to overkill with a wetsuit. There's usually enough water flow to provide natural cooling unless it's a semi-dry or very tight fitting. In the worst case, one can always admit water by stretching the suit under the neck, the same way one would flush the suit after peeing.
I personally use a full 3mm suit. A shorty might keep most of you warm, but the full coverage will protect you better against hydroids and fire corals in case you're going down low to take a pic or look at something small. The "floating hydroids" another poster mentioned are probably siphonophores and they can deliver a little sting as well. For many, it's not the initial sting that's bothersome, more so the itching rash that often develops after. And, in the unlikely but possibly case you get pushed into live coral, a thicker suit with full coverage can protect against coral cuts. Finally, while you probably won't experience one without seeing it first, lionfish are present in the local waters and scorpionfish lie camouflaged on the reefs. A 3mm suit may not provide much protection against the spines, but you probably have a better chance of avoiding injury than with bare skin alone.
