I have to tell you that you need to check that your "splitter" is good or not. They can blow with too much current being pulled. Disconnect both boats and the check that you have voltage at each output with a multimeter.
If both outputs have voltage, reverse the connections and see if the other boat is experiencing the same problem. Then the problem is in the splitter. If it not then the problem is in your boats electrical system.
Then check your 110-VDC breaker panel and make sure the breakers are OK, and you have voltage from all of them.