Work permit - yacht rigger

A query about yacht crew working in UK waters.

The "seamen" exemption seems to cover paid non-EEA crew arriving to take part in an international yachting event out of a UK harbour.

But does a non-EEA crew member need a work permit for work confined to the harbour (such as rigging), or for crewing a boat within UK waters?

For a US national who does a mix of paid and recreational crewing for various boats, in and out of port and coming in and out of the UK regularly, what's the most efficient visa application to make?

Any pointers gratefully received!