What sort of boat are you looking for? A small wooden rowing boat, or a
larger motor vessel? Civilian or military? (I assume by "boat" you're
referring to a small vessel, and not a large ship)
There are very few 1/48 boat kits available in injection-moulded
plastic, and most of these are military landing craft (HobbyBoss have
just released a LCM-3) but there are quite a few "box-scale" kits which
are close enough. Revell have recently reissued their "Sports Fishing
Boat" in 1/55 scale, which is a Chris Craft cabin cruiser (and could
easily be modified to represent a civilian river or sea craft) and
Lindberg have produced a number of small motor vessels in 1/60 scale
and thereabouts, including a tug, fire boat, tow boat and a number of
fishing boats. (though most, or all, of these are now out of
production).
If it's a military vessel you're looking for, Revell/Monogram have
produced a Vietnam-era PCF Swift Boat and RAG river gunboat in 1/48
scale. Lindberg also made a PT boat and Air Force rescue launch in 1/60.
As for sailing boats, there are a number of yachts in the region of
1/50 scale, including two kits of Polish racing yachts produced by
Mirage, both of which are currently widely available.
If you're looking for a small rowing boat, I think these are only
available from specialist resin manufacturers, though you might be able
to use a "ship's boat" from one of the aforementioned kits! I think
Billing Boats (and other manufacturers?) also produce plastic rowing
boats as part of their accessory range.