Tempered all the way. Forget about Starphire for this run imo. It's way more money and weaker. They don't call it low iron content glass for nothing
I don't even think they can temper Starphire. Starphire is just a brand name for low iron glass.
I have seen many ply wood tanks. The best ones i have seen are 2x4 framed and braced to the hilt. And those seep also. There is not much salt water cannot destroy. Go to a boat yard and inspect a few old boats. You might even get some better ideas and consider fiber glass. It will last a bit longer that is for sure. Hate to bash this early on but everyones rooting you on without thinking of your safety. You understand how much pressure and weight 300 gallons is i hope. This is not something i would build hammered either. Alcohol will impair your skills i don't care what anyone says.