This activity works for babies (touch and feel), all the way to elementary aged kids! You have lots of options...
*Paint the ocean
*Paper towel paint
*color the ocean w/ markers
*Use the side of a peeled crayon and make the water
Once the water has been drawn/painted, start adding details...
*We added shells collected from a previous vacation
*spread glue on the bottom (we used fingers) and sprinkle w/ sand
*Add glitter to wet paint/glue for a sparkly effect
*Use pom-poms for anemone, collected sticks for seaweed (or ribbon, pipe cleaners, etc)
*Make fish out of construction paper to glue down, or use fish stickers
*Finally, stretch out some cotton balls for clouds.
The idea is to use as many sensory (Touch and feel) items as possible. Dig around drawers, craft boxes and see what could substitute for items under the sea. If I'd had green streamers, I would have added those for sea weed.
For young children, make a small version yourself and let your child touch it, and talk about how it feels (scratchy, rough, smooth, cool, soft, etc).