These tips pertain to seaside activities. They are general in nature and in exhaustive. 

On the ship 
If you are traveling by ship, keep yourself physically occupied to prevent seasickness.
Most of the native islanders transit by ship. Befriend them during your journey. It is an ideal opportunity to know more about your destination.
Learn how to use a life jacket if you are traveling in a ship or a boat. 
Get thorough info from the locals about the weather before you venture out into the sea or even if it’s a seashore visit.

While snorkeling 
While snorkeling be wary of undersea currents.
Avoid touching marine life. They are fragile and sometimes poisonous, even if dead.
While enjoying snorkeling, involuntarily you may have floated away into the open sea. So keep tab of your distance time and again.
Get thorough info from the locals about the weather before you venture out into the sea or even if it’s a seashore visit.

Seaside camping

Understand the tides before you campout on the seaside.

Carry tents that do not need pegging. Peg does not hold on to loose sand.
During low tides small marine life gets stranded amongst shallow rock. It is the right time to go looking for them.
Do not touch any marine life. Some creatures sting and are painful, even the dead ones!
Generous use of sunscreen cream recommended.
Never collect or buy corals, shells or any other form of marine life. Be an eco friend.
Never walk bear foot on rocky seaside. The surface is slippery and sharp.
Seaside has sultry weather. Drink lots of water