Step one, invest that 300K in 90 day term deposits
Take 90 days to buy the boat, ugly but solid, with no brand loyalty or preference, just the best ugly bluewater boat I can find for 75K.
Split 50K into 10K parcels, and invest in 30 day, 60 day, 90 day, and 120 day term deposits, and keep 10 k for cash. that 10K cash is the phase one refit budget. as each 10k note is freed up, it should dovetail with the next refit phase. If not, roll the cash over for another 30 days. you ain't getting rich, but you ain't losing a dime, and you are not spending it, cuz you can't touch it. The remaining 175K invest in in a combo of stocks, bonds, 12 month GICs , etc.
Quit your job- I don't care who you are, but if you are on a tight budget, chances are your current hourly wage is a hell of a lot less than the $90-150/hr that the local yard is charging to do stuff on your boat that you could probably do yourself if you weren't working. The money you save on labour costs will cover your expenses, cuz after all, you got the 300K by selling your house, so you have no mortgage, no car payment, etc., cuz you paid all that stuff off, right? If you didn't, go back and rethink step 1 and step 2, cuz your plan is different than mine.
Besides I'd rather be working on my boat than actually working.
In 6 months you are ready to launch for ports unknown, with a nice shiny looking reliable well equipped safe old boat that you know inside and out, and enough cash to cruise indefinitely, if you are frugal.