Aquarium
Put an Aquarium in your room full of fish. Children love to watch the fish swim
around. Hint: Don't use a salt water aquarium as they are hard to take care
of. 
Dry Seaweed
Put seaweed (purchase from the grocery store) in your water table. This makes
the area smell like the sea. The seaweed also feels like the real thing.
Sea Habitat
Put things that exist in a sea habitat (sand, small shells, smooth stones, seaweed,
etc) for your children to look at. 
Giant Sea Shell
Get a large magnifying glass and let the children look at sea shells with it.
They will be impressed with the make-up of sea shells
Difference in Water Types
Have two bowls of water. One saltwater and one fresh let your children taste
and smell the different waters. Then let them float different items in the different
waters. (Items that will not float in regular water will float in salt water)
Finally, put two small amount in the freezer and check on them every couple
of hours (the salt water will freeze slower)