I visit Thailand frequently on 30-day visa exempt entry. I own a motorcycle here. When I bought the insurance from Falcon Insurance via a Kawasaki dealer the bike salesman told me that I must have a Thai motorcycle license for the insurance to be valid. I used to have a 1-year "temporary" Thai license but it has lapsed and I just have my overseas license now. As a non-resident (tourist on a visa exempt status) I presumably do not need to have a Thai license. Is my insurance valid?

That would depend on the policy conditions. Check those and you'll know.

How can I find out if my UK boat driving licence (boats up to 48 feet and unlimited horsepower) is valid in Thailand? Thanks...

As a frequent visitor why not get a Thai driving license...not difficult and will assure your confidence. Get a double-double while you're at it. Both are inexpensive and the tests are not taxing. Never know when you'll need to rent a car. Good Luck