Definitely go with 1 1/2 tube, have 2 vertical members to split length in 3rds and if they build it right it will hold. And by build it right I mean, build the front and back frames first with 45 degree corners and then join them with the horizontal members. Doing this makes your corners bullet proof. You could add a knee brace into the backside of the stand just to be really on the safe side.

Personally I would still suggest going 2" for the amount of weight. If you do use 1-1/2 then be sure it is actually HSS (structural steel, cold forged). If it's just hot rolled square tubing 1-1/2 is going to be pretty sketchy. As others have suggested extra columns is a good idea as well as adding angled supports at all corners to reduce the load on the joints and keep things square.

Good point almost all tubing 1.5 or smaller isn't tested for yield and tensile with the exception Griz mentioned 188 wall. Personally I would worry more about the guy doing the fabricating. You could get the best materials but if the guy doing the work doesn't know what he's doing it could be a disaster

Are those popsicle grade or fudgsicle grade sticks? You might need some cross bracing in there, esp if you're gonna use a wavemaker