the diffrence

Hello!!

What is the difference between these phrases?

travelling IN sea
travelling ON sea [ as in travelling on sea ice or sea routes]
traveling BY sea

Thank you

The first doesn't really make sense ('travelling in THE sea' would mean travelling underwater); in the second, it would depend on the situation ('travelling on sea' isn't correct, 'travelling on sea ice' means on top of the ice, 'travelling on sea routes' would mean to travel along the route line, etc.); 'travelling by sea' would mean to travel in a ship or boat.