They are fine at lower volumes but obviously that is not what they are designed to do. Think about boating in general, there are very few times you will play your stereo at lower volumes. When I do not want to disturb my surroundings I just turn the tower speakers off. They sound fine at low volumes but they still carry across the lake much more than the boat speakers.
Are you still running your "budget" wakeboard setup?
If yes, you might consider a few other options. I/O's are much quieter than high-end wakeboard boats with thru-hull exhausts. My buddy ran some Precision Metal Works cans loaded with Infinity's on his I/O and it was decent. You could hear it OK when you rode. He had about $400 into his tower speakers. You might also consider the Evids 6.2 for your boat. I just picked up a set for my backyard for a little over $200 off E-bay. They sell all day long on E-bay for $300/pair, clamps go for about $50/pair.
If you are making an investment for the long term you are wise to think HLCDs. I would say the Rubi 350's are my least favorites. Another buddy has a Rubi 450 in the middle of his tower and a pair of 350's on either side. When I compare his setup to my Wetsounds or other NVS systems from other friends I would rank them third.