The output of 3x80w or 2x120w panels will be very similar.
More panels of the same total wattage do better in conditions of isolated shadows. These conditions are common on a boat so the 3x80w will be slightly better all other things being equal.
However, because of losses around the frame etc the 3x80w option will be a bit larger in surface area with the same cell effeciency.
The difference in output between the two options is small so generally ease of mounting, getting the appropriate total size, and cost, are the important factors.
When looking at cost make sure you factor in shipping
charges. Some couriers charge an extra insurance
cost on glass panels above a certain size. So 2x120w can have a significantly higher delivery
cost than 3x80w. This difference is easy to miss as the delivery
costs are not usually shown on the website prices.
The flaw in your thinking is that the amps per cell is not constant it goes up a wattage increases (assuming the same Vmp). As Olorin says watts are watts. Fit the most watts you can squeeze in.