Nathan;
I actually DIDN'T have good luck with the splice (I used 6 wire thermostat line as the LEDs I was going to use contained 4 different lines due to the various flash patterns) and undid the whole thing because the lights were too dim. Not sure how this would work with the underwater lights, but if adding line increases resistance, you're going to lose power to the lights, I'm thinking. That said, as I said, you can try splicing one section and see how it goes. The waterproofing might be an issue or you can use some sort of shrink wrap mechanism (or even encase with silicone glue, etc), but three feet between lights looks nice.
The units look/feel pretty decent for the price, imo, and as I said, they've worked for 3 months or so of 2011 and are working now, having stored them in the basement all winter. Wasn't sure they could take the freeze so I opted to take caution's side. Btw, there are more sets (I think he sells in 15 lite sets too, but you'd have to check out his 'ebay store' and do some nosing around. Could even shoot him an email and ask, if you were looking for more quantity; no doubt the price per unit would come down even more.)
Glad I could help.
Michael