Some folks will go off talking about having as many fine strands as possible, to take advantage of "skin effect" for better frequency response. While you can maybe measure it, and while some folks might hear a difference in controlled situations, this is a wakeboard tower application, and you likely will not hear the difference. Lots of very fine strands does make a much more flexible cable and it is more resistant to breaking, but for a permanent install, I am not sure that is important.
Lots of trailer wiring has fewer strands; it is stiffer for the application. It would work, but I personally would not use it. I would avoid it due more to its stiffness than any perceived audio quality.
I know that the tinned mast wire, if it is really marine wiring, will last you a long time for sure. MOst mast wire seems to me to be finer stranded, and more flexible. The tinning really helps with corrosion resistance. I see no reason why you should not consider using it, especially if you feel you are getting it at a good deal. 14 gage will be appropriate for most but the very high-power applications.
I say use the mast wire, and don;t worry 'bout it!
For most speaker wire there is nothing magical about it that sets it apart from other multi-strand wires.