Can you get a visa on arrival if you arrive by sea in Bangkok or must you get it beforehand.?

It is actually a visa exemption. Visas are issued by embies and consulates. You get a visa upon arrival when you enter Thailand. Doesn't matter which way you do that. What will differ is how many days you get. If by air, you get 30 days. If by land (a port = land) you get 15 days.