Our boat sometimes puts out air bubbles that come to the surface alongside the hull even with the shaft tunnel exits. Our shaft logs do not drip when the shafts are not turning (actually, they rarely drip when they are turning). In looking at the little clear debris filters that are in the raw water pickoff line from the heat exchanger that connects to the shaft log one can see agitation bubbles in the water swirling through the filter. It is these bubbles that are being pumped to the shaft log and out the shaft tunnel.
In other words, it's no big deal.
Our bilge is essentially dry although over time it gets some rainwater in it from the lazarette and also the very slow drips from the shaft logs when the boat is underway. So we vacuum it out every four or five months--- there will be perhaps a half inch of water at the lowest point.
So I agree with Mike. It may very well be that the bubbles are just the nature of of the beast. Sources of salt water in the bilge can be varied. Ones I can think of offhand are:
1. The normal drip from the shaft log when the boat is underway. (Ideally the log shouldn't drip when the shaft is not turning. Adjusting the packing gland is real easy--- lots of descriptions in the archives of this forum.)
2. A leak in the (or an) engine's raw water cooling system. This can be a worn seal in the pump, a seeping hose connection, worn gasket under the cover of a sea strainer, a leaking heat exchanger, etc. This same applies to a raw water cooled generator.
3. A seeping seacock. There are lots of different kinds of seacocks. Our 1970s vintage seacocks are big, bronze, and the fit of the rotating cone in the seacock body can be adjusted by an adjustment and jam nut arrangement similar to a packing gland. Tighten the cone too much and the seacock will be very hard to operate but it won't leak. Adjust the fit too loose and it will be real easy to operate but it will seep water. What you want is the cone to be a tight enough fit not to leak water but not so tight that it's difficult to operate. Like a packing gland, it's a trial and error deal.
4. Leaking wet-exhaust hose. Either from a not-tight-enough connection to a muffler or through-hull exhaust fitting or a bad spot in the hose.
5. Leaking rudder gland.
6. Leaking intake water connection to a salt water toilet.
7. Leaking connection to a salt water washdown pump.
There are probably other potential salt water leak sources but that's what I can come up with now.
and hooligans 
