This unit basailly looks the same but with different maps. Assuming this is just a marine version of the 66i.
Not the same..... Body is longer (taller), for example....
There are also interfaces with other marine hardware. This is definitely a specialized boating device, not a general-purpose handheld.
But it is also useful for hiking/outdoors and in combination with kayaking etc great, because it floats (I think this is the reason for the different body design).