Would be great to have the furnace boats require new coal after a certain amount of in-game time so they are not overpowered for water transportation.

e.g. Just an idea....the equivalent energy of a coal block could be left in for two full day-night cycles. 1 piece of coal/energy needed for a 5 minutes of in-game time (25% of 1 day), or 4 coal pieces for a full 20 minute day-night cycle.

It is consumed exactly like the furnace minecart consumes its fuel