The problem with the ancient marine is the lack of informations. Actually there is only a few boats found that could be celtic-like:
The Dover Bronze Age Boat (long for 8-15 men). The Ferriby Boats (from 6 to 20 men). The Hanson Log Boat (long for 1 or 2 men). The Flag fen Early Iron Age Logboat (long for 1 or 2 men). The Canewdon Paddle (long for 2-4 men). The Hjortspring boat (long for 12-20 men). The Arles Rhône 3 Gallo-Roman 1st century AD (31 meters of long).
Edit: @GunChleoc I ch