Mountain bikes take up a lot of room so you will probably have them lashed to the stanchions outside. This means they will always be coated in salt
spray and will rust apart in a year or so. Many cruisers carry the Denon folding bikes and like those.
But it all depends on where you cruise
. If you are just bumping around marinas
in the US, bikes are a great way to get around. Go off the beaten track into the developing world and you will find them much less useful. Roads are dirt, the locals walk or hitch and you will do the same to meet them. Riding a bike in a country that no one else does will just put you that much further out of reach of meaningful contact and understanding of local culture and local life.
Leave the bikes at home.