Aww, my specialty. (however it's a very simple calc)
P (power in watts) = I (Current) x E (Voltage) -or-
I = P/E
if your voltage is 12.5 nominal, and your power is 50 watts, you can simply divide the power by the voltage to find your ID.
50.4/12.5 = 4.03 amps x 2 = 8.06 total current consumption.
So, if the voltage fluxuates, you will have slightly different numbers, that will slightly deviate, but if it's 10 amp breaker, you might want to go slightly larger in size, maybe a 15.
If the lights flex a little, could be a possible short inside of the cable when it moves. If you are blowing a 15 amp or larger breaker, check the wiring and make sure nothing is shorting out when the boat moves.
The lights inside are "Grain-o-wheat" lamps and are small, but definitely not LEDs as mentioned before. Cool idea though! I have 1/2 billion white LEDs around our shop, I might just build a custom setup for the lighting with dimmer control before the boat is taken out of storage. Some of the pictures I've seen are impressive!
One thing I don't get is it says they will operate from 12VDC, or 12VAC. If they are meant to operate off of a separate transformer for 12 VAC, the power consumption can be quite different. This means that the bulb is rated at a higher overall voltage and the entire calc will change. Do these come with some other transformer or am I missing something? Look at the package where it says it has to use a UL rated outdoor transformer... which leads me to believe that it's a 12VAC rope light.