If you're sitting in one place siphoning, you're going to be a shivering a bit by the end of the 1st hour dive. Go with the 7mil, no hood, gloves optional. If you're not going to be working and just doing a guest experience dive, you'll probably be in short enough to be comfortable in the 3mm w/ hooded vest.
Some aquariums require you to use their equipment. Scuba gear for liability reasons and contamination, exposure protection for the latter reason.
Open circuit Aquariums often don't need to worry about contamination as they are constantly pumping water from the ocean. Close circuits however do have to worry about that issue.