A late arrival, but I'll chime in.
The side of the coaming would be typical.
And consider the long haul ... a 23 may or may not be surveyed at (eventual) sale. There are many requirements regarding grounding (not the keel vs. mud type), bonding, and circuit protection; particularly the distance from the inlet to the first circuit protection (measured along the length of the wire) must be 120" or less. If that's your panel board, fine. If not, then there needs to be a circuit breaker within 10' of the inlet ...
Thanks for your response moonlight! I installed a new 30 amp smartplug inlet on the port coaming and ran 10/3 less than 48" from it to a new Blue Sea (main + 2) 360 Panel System. Will this meet mentioned grounding requirements?