I have a (year) 2000 38 Ocean and, to date, have had no problems with water in the bilge – in fact it has been the driest boat I’ve owned – so your problem is not typical.
Where is the water exactly and are you sure it is seawater from around the keel bolt? I have had fresh water in the engine compartment from poor sealing around the cockpit drains and also around the instrument binnacle. The water actually collects at the front of the engine over the keel bolts, so the source was not immediately obvious.
If it is the keel bolts that are leaking, is there any evidence of the keel ever having received a hard knock (e.g. hitting a rock or otherwise grounding)? If so, I would suggest you need to get a survey by an expert (but I can’t recommend anyone).
If not, it might be worth trying a temporary ‘repair’ (dropping the keel to effect a permanent repair is not a trivial job). You could remove the offending nut and use epoxy putty (which sets even in the presence of water) under the washer, around the thread (but not too much otherwise you’ll never get the nut off again when the time comes) and then torque the nut up as hard as you reasonably can. I don’t think this will make the problem any worse.
If the boat is out of the water (and the threads dry) then you can substitute silicone sealant for the epoxy putty.
I think any attempt to seal around the keel to fibreglass joint on the outside, without dropping the keel and doing a ‘proper’ job, is unlikely to succeed.
PS Did you have a full survey carried out before you bought the boat? If so, you might have some claim against the surveyor.