Some days it's not going to be an option to go to the beach. Some people may only have access to pools in the summer time if beaches are too far away. As most states are relatively far from the ocean, and don't offer many lakes for swimming or water play.
(make sure there are no allergies to shell fish)
If going to the beach is not an option, make your own! Most craft stores have sea shells you can buy, or if going to the ocean, collect them together. Even most grocery stores may have some different types of shells. Fill a water table or long tote with water, and put the different shells, and containers in the bucket. Sand toys work well as water toys too. Rubber ducks, water toys for the tub including turtles, and mermaids in the bucket of water.
In a different area set up a sand box with beach sand you can find at Walmart, local hardware store, or pool supply store.
An important sensory aspect of the beach is the dry, soft sand. Sand has a very different texture than water does. Having a sand box filled with sea shells and different sand toys to aid in explorations of textures include; shovels, scoops, spoons, bowls, buckets, sifters, and shaped castle buckets.
The children can either go back and forth or choose which area they like the most. Have towels ready, swimmers and swim suits on for the water table. I suggest having a change of clothes handy and lots of towels. Make sure sand stays away from mouths, and they do not drink the water. Babies should always be supervised while near water. The tote/table is not for swimming. Always take caution with babies of any age, near any kind of water.