mopower

One or two 100 watt bulbs are usually fine for short periods of mid 20s ( a few hours overnite) in a closed up engine compartment.
If you cover covers the vents better yet

wrh3

Lightbulbs in a bilge compartment........recipe for failure and disaster, look at some of the pics on BoatsUS.com of others that used these. Lightbulbs can crack and it doesn't take a lot to ignite gas vapor.......better off to buy a heater designed for a marine bilge compartment, Boatsafe or XtremeHeaters.com make one.......I know guys that have used them for yeasrs with no problems but the what if would bother me.....the electric heaters are subject to not running if power failure occurs, I guess I have always been close enough to the dock and known when the power went out so it was not a problem.

xtremeheaters

These heaters get around 300 c and that is deadly around gas
gas will ignite at 250 c and the thermostat will set out an arc that is very dangerous
please look her http://www.xtremeheaters.com/tips.htm

DirtyMoney

I use to keep a light on in my well house and it would either burn out unexpected or would not be enough to keep up with some cold nights and I would be outside heating pipes trying to thaw with out busting them.

mjpcowboy

Spend the money and buy a heater designed for heating the bilge. They are a few hundred dollars but then again how much is your motor(s). I use one at LOTO to get a few more weeks on each side of the season and it has worked very well so far.