One of the outcomes of growing up in a land-locked state. I adore the ocean (and rivers, lakes and creeks, too). There is a lot of variety in beaches and oceans, which I like alot.
Belize: This tiny Central American nation has the second largest barrier reef in the world (the Great Barrier Reef of Australia is the largest). The snorkeling and scuba diving are world class. The beaches themselves are rough and rocky. If you stay on Ambergris Caye, go to one of the reputable dive shops and book a snorkel and picnic trip. A couple guides will take a group out snorkeling, catch fish with spears while snorkeling, and then stop at a beach where they grill up the fish for a tasty lunch. After a lunch and siesta, you can go snorkel some more. By the way, if you do travel to Belize, I recommend you eat and drink their specialities: tropical fruits, fish, seafood, beer and rum.

The tail of a Northern Right Whale
Boston, MA: If you're in Boston, take a day and an inexpensive water shuttle out to the Boston Harbor Islands. Area of fresh air, and a nifty military fort. Go whale watching. If you have a longer day, take the commuter rail (or drive) to the fishing village of Glouchester. There are numerous restaurants serving fresh lobster, and nice shopping areas. The beaches are great to relax at, although the water is usually a bit too cold for much swimming.
Dover, England: Turns out that the Brits actually think Dover is just another commuter town and scoff at tourists who go there. But in fact, it's a delightful place. And well worth visiting for Dover Castle and the secret wartime tunnels - where you actually walk inside the Cliffs of Dover. Also a good spot to take a shuttle over to Calais or other French commuter towns over the English Channel.

Kayaks on the wharf, Monterey
Monterey Bay, California: Highly recommended: spend a day in the Monterey Bay Aquarium. Go kayaking on the Bay (you can often see sea otters playing, eating or sleeping in the kelp beds). Walk and picnic along the beaches. Rent a bike and and check out the routes between Carmel and Monterey. Shop in the area's many thrift shops.
Oregon: Bandon by the Sea. Just go there. Eat the fish. Walk along the seashore. Watch the furious waves. Yum.
Duluth, MN and the North Shore: Lake Superior is so large that it looks like the ocean. In fact, storms have caused ships to be shipwrecked on this lake. There are many fun things in this region - especially in their brief summer - drink lemonade or beer at the many bars that overlook the Lake, visit the aquarium, hike in the cliffs and woods along the lake.