I've had one installed for a few seasons. IMHO get this one and wire it to a constant power source.... the battery. It has an on/off switch. With a setup like this your sled doesn't have to be running to charge a device.
Just remember if you "direct wire" you need a fuse near the battery and an on/off switch as the process of changing from 12 volts down to 5 volts will drain a battery that is not being charged. (The USB port is not going to like to get wet, does not matter what "waterproof" statement is made, getting inside of the USB port wet will fail over time but minimal cost so who really cares other than add it to your pre-season check list)
It has a built in on off switch. I transitioned to this one because of it's built in power switch. Previous versions that I used required the sled to be running in order to charge a device, leading to engine overheating issues. And yes, fuse protect the circuit.
On the water issue. I've never had a failure due to water intrusion. All failures were the result of vibration causing an internal component of the charging port to separate from the circuit board.
This problem can be avoided by disassembling the unit and adding a small amount of epoxy carefully to that component, but that is beyond the scope of the OP's question.
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