Gravitational potential energy
No, the tides do not store energy. The energy to be 'harvested' from the tides is kinetic energy (movement energy).
in the sea
During spring tides, the sea comes in far up the beach, and goes out a long way.During neap tides, the sea doesn't come in as high up the beach, or goes out as far.
Sea levels rise with the tides.
In high tides areas, the water level is higher the average sea level. In areas between high tides, low tides from. In low tide areas, the water level is lower than average sea level.
the moons gravity
Tides
High tides ocurr every where, on every sea coast harbour etc
gathering of sea shellfish at the seashore
Yes
The sea floor channels the water in some places making tides higher there. Obviously, "at sea" the sea floor is too far down to be channelling much of anything. Also, if you're "at sea", how do you even notice the tides? It's not like there's anything sticking out of the water to compare it too.
All the seas have tides, because the moon's gravity affects all of them - but to a different extent.