That depends on what type of usage you have in mind: For example, if you just want something to get you around the marina, 1.5 - 3.0 hp would be fine. If you want power on the open ocean when the winds are gone, you'll want something that is large enough to make headway without rattling the spine to pieces.
25:1
The 25 hp Johnson outboard runs a plug gap of .030 in.
to of 25 km/h
2500
25:1
20/1
1990
A 25 hp Mercury outboard, serial number 6459040, is a 1984 year model.
A 25 hp Mercury outboard, serial number 5848745, is a 1981 year model.
40:1
50:1
The required plug gap for a 25 hp Johnson outboard is .030 in.