Originally Posted by Augie
At that distance, #8 copper is adequate, just, for a 15a circuit.
#12 copper will do from your on-shore j-box to the end of your dock.

Would be fine for lighting, stereo, fountain pump, battery charger, etc.

Add power tools into the mix you'll need a 20a circuit and #6 copper to maintain <5% voltage drop.

If there's a need to maintain <3% voltage drop you'll need to go up one conductor size on both of your runs.

For an install such as this I would recommend using a GFCI circuit breaker in your load center.
GFCI outlets offer excellent protection, but they don't like the damp conditions that are present in outdoor installations.

I ran power from the outbuilding to the pier, and put a load panel out there with a GFCI for a hot tub out there. I figured if it was rated for being outside near a hot tub, it should work. It tripped once during a thunderstorm and it took me 2 days to figure out why the pump in the pond wasn't pumping. I had forgotten all about the GFCI.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).