Having lived at the beach for 33 years and owned a vacation rental house for 18 years, and having been a fire department chaplain for 16 years I've figured out some safety tips for having children around the water.
The number 1 priority when taking kids to the beach or staying at at beachfront home or one with a pool is to always have someone watching. By watching I mean literally watching. Being present isn't enough. Put down the book, the phone and face the ocean, lake or pool and stay on top of where the little ones are located. Forget the nap. They can take off down the beach quickly.
Residential pools are surrounded (by code for your protection) with a fence. If you can enter the pool area directly from the house, keep the door locked when there's not an adult at the pool. If there is a gate, it should not be accessible by a small child. And DO NOT prop the gate open for convenience. It's there for a purpose...to protect your children.
And if you have no children at your vacation rental/waterfront home you still need to keep the gate closed and locked. A child from next door could wander over and into your pool.
A cover on a hot tub should only be removed when an adult is present and
in the tub. Little ones not only don't belong in a hot tub, but can
fall into one left uncovered.
The first thing you want to do when arriving at your rental is to unpack. But if you have children, assign one adult to stay with them. Kids love to explore, and the first thing they want to see is the pool. On more than one occasion with which I'm familiar a child has drowned while the adults were unloading the cars.
At the beach it's not the lifeguard's job (you are swimming at a lifeguarded beach, right?) to babysit your kids. While they keep their eyes on the water, it's your job to protect your child. And always ask the guard about rip currents and tides. And outgoing tide (when it changes from high tide to low) can pull a small one out. It's not a lake or the pond on Grandpa's farm.
Life jackets save lives.
It only takes a couple of seconds for them to wander into danger. Don't let those seconds become the worst of your life. The pool or the beach can be the best part of your vacation. But it can also become your worst nightmare.
I'm sure there are probably some more I can think of, but this will get it started.