This one may be a bit tough to answer…

First, depending on what type 29 foot cabin cruiser (lots of big windows or not) a 10 K could be marginal in the climate you describe, but if you have had the same combo in the same climate in the last 3 years (age of unit) and it did ok, then we can take it from there…

Next I have to admit that I have not seen an Ocean Breeze unit in the last 3 years to know what type control they are using, or how they are utilizing a given controls protections…

Third…I think the advise you got from factory reps is…Well quite frankly…Absurd…For a factory rep to tell a customer to stick a pen into a schrader valve to let an ozone depleting gas out into the atmosphere is not only illegal, but could cost him much more than his job…

That reason aside…Counting to a count of seconds while letting off that gas tells you virtually nothing (except how to count) about how much refrigerant you have let out…IE: Which schrader valve (high or low side) did he say to let it out from ?  Should the unit be running or not while letting off this gas ?

Not knowing the control you have…Ocean Breeze shows two on their site (Both are from Micro Air & both were used by Marine Air in the past) that I am familiar with…One is a very old Micro Air version that we almost never see in the field anymore, and the other is also one that Marine Air used from 1993 thru 1997…From your description I'm guessing it is the older of the two models, and I think I have a fairly educated guess as to what was / is happening with your unit…

As I have described here in other posts & articles…It's not just seawater flow (although flow is paramount) but also the condition of that water coil, and it's ability to transfer heat to the water…At 3 years of age it's likely in need of a acid flush to remove the scale blanket that builds up over time inside that coil that is acting like an insulating blanket preventing proper heat transfer to the water…Yes this could cause the original shutdown you mention, and the seawater temps you quote can make this worse…

Yes…Letting freon out can lower the head pressure & keep the unit running, but it's not the answer & doing so also lowers the low side pressure thus causing the icing you describe…You solved one problem & caused another two…Icing is one, and compressor overheating is another…

My suggestion is to either flush the water coil yourself, or just get a qualified guy out to both flush the coil, and replace the refrigerant that was let out…You are going to need an experienced guy that has the equipment to charge the unit anyway, and it shouldn't take him much longer to do the flush…Make sure he knows that you let out gas, and to do the flush first…If he adds gas before flushing the coil…He may need to add more after the flush as the condenser will perform better, and the unit will again seem low on refrigerant…

It also may be one way to tell how experienced he is 

Again…Not that I don't believe you…But a factory rep telling you to let out gas is unbelievable.

Steve~