I think I'd like a couple more metal cross braces underneath that top: if wood, and it gets wet, it gets soft. You can get pipe and clamps and such at Lowes or HD or ACE and should be able to make it better. Also, triangular braces on all the corners would increase its strength considerably. I don't know where you are, but quake or just somebody leaning against a side could skew those braces dangerously. Salt water is about 8 lbs a gallon and that's a lot of weight...you know what happens to a vertical/horizontal frame structure if it gets a sideways force before it gets cross-braced: it tends to deform and collapse.