Sure sounds like a defective battery. Over here, any auto parts store will do a free load test and confirm if it's bad.
If you want to test yourself, just check voltage with a meter right at the battery terminals. If the voltage dropped at the battery, then you have eliminated all the connections as the possible problem.
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"Knot Easy" 2000 Horizon 240 Volvo 5.7GS /SX
tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings