Mercury 115hp is the lightest 115hp on the market, also probably the cheapest, it has a great reputation and is the only one made in the US by a US company.
I have never weighed it but it is listed by the manufacturer as 1650lbs, the 90hp I have pushes it to 36mph, cruises in the low 20s pretty well which is more than fast enough for where I use it as it is usually pretty choppy.
I have a 19' skiff with .250 bottom and .1875 sides, it is still a pretty light boat and goes well with a Honda 90, very tough, while it may be overkill for some applications given the choice I would take overkill every time.