We have been using our "Undersea University" science kit that we got from Scholastic.  I'm amazed at all the information that is provided and covered, along with a lot of hands on activities.  We've read about and discussed:

Marine Archaeology

Marine Geology

Marine Ecology

Ocean Exploration

Marine Biology

Difference between oceans and seas

Early oceans and how they took shape

Properties of water

Crustations

Colors of the oceans

Salt water

Why waves crash

Tides

Currents - surface and deepwater

Rain

Hurricanes

The food web

Plankton and plankton eaters

Predators:  The meat eaters

Ocean exploration

Shipwrecks

Sonar

Reefs

Along with these, we've also been doing the experiments included in the kit.  These include:

Finding the percentage of times your finger will land on a spinning globe.  Each child did it 7 out of 10 times, or 70%, which is how much of the Earth is covered in water.

Growing our own Triops.  From preparing the water, hatching the egg, feeding it properly and transferring it to the tank.  The best part was watching how fast it grew, from a speck of dust to 2 inches long in just 3 weeks.

Made tides in a shallow pan.

Made currents in a shallow pan.

Made our own rain.

Played card game:  Gulp War

Made a mini-sub to see how one can dive and rise.

Made our own Sonar using a box and various items inside and graph paper.

All three of the children participated in this project together, it took about a month to cover everything in it.  We have two more packets ready to use once this one is complete.  The kids really enjoyed it and I feel very confident about getting any of the other Scholastic kits.  It has also inspired them to check other other books from the library and various movies from the library and Netflix.