The easy way MIGHT be...
To just replace the vent hose. However, before doing that, there are a few things you can try that don't require rubber gloves or any surgery. First, I'd make sure that it isn't just the thru-hull that's blocked. If it's the typical "vent fitting" that's a cap with a slit in it, it's the tightest spot and the one most likely to clog first. Blast it with the hose under as much pressure as you can generate. If that doesn't clear it, several people have told me that they've been able to clear a clog by blasting with an air horn. Be careful doing that one...if you succeed in clearing the clog that way, blasting away too long could pressurize your tank! If a couple of short bursts won't do it, quit. You might be able to clear it simply by sticking a hose (NOT the one used to fill fresh water tanks!) down the pumpout deck fitting and washing the clog out...just let the tank fill till it forces enough water out the vent--which is what you should have done immediately instead letting it stay clogged until the clog may be so dry and impacted that nothing will open it up. And next time, pay more attention to how full your tank is getting? The vent is an AIR vent, not an overflow. If heeling causes it run out the vent, the vent should be relocated on the tank to the inboard side of it, so that it's always an uphill run, no matter which way you're heeled. It can be done, even on a plastic tank...instructions on request.
