I have an O’neil wetsuit designed for surfing I use for packrafting. It’s a full-length suit, I believe a “Heat” model, around 5mm I think.
I use it in Washington state, in mixed rain/snowmelt rivers. Overall, it’s been pretty good, and I’m happy with it. However, one day I spent a couple hours in a cold river on a rainy, chilly day, and had a good swim at the end in C III / IV water, and got cold fast.
My conclusion: good in sunny NW conditions, but on a drizzly, long day, it left me dressed for the paddle but not for the swim. The big defining element that’s emerged for me is that it’s great if I’m very active, but the longer the run and the less active I am during, the colder I got - so sustained activity makes it adequate in chilly conditions, but when I’m drifting or just plain out long enough to build a significant heat deficit, a drysuit or a 7mm starts looking really good.
In better conditions, though, I spent 8 hours in it at night, in rain, in Seattle in high summer, including a bunch of swamp-swims.
I also wear it for a lot of testing boats, including jumping in & out in cold water.