Koh Larn is about 10 minutes by speedboat from Bali Hai pier at the southern end of Walking Street in Pattaya. Buy boat tickets there, either speedboat or slowboat.

Not what you'd call an unspoilt relaxing place to lie on the beach. Hundreds of deckchairs and beach umbreallas, dozens of jetskis & banana boats for rent. And more speedboats in one place than anywhere else in Thailand.

Some nice views and you can walk a bit and get away from the crowds, also there are some small areas designated for swimming - but unless you are Chinese or Russian and on your first package tour to Thailand you probably wont enjoy it much.