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.
