I had a mercruiser 330 run 65 on the gun up river with a 21' crusader pickle with an A/T.
A 330 is a 454 10.25-1 .488 lift cam and a large set of oval port heads. Atleast that is what was in it when I tore it down. I believe that it was an out of the box 330 mercruiser. I also had a tunnel ram with two 660's on it. I probably could have gained a little with a single carb and a large dual plane intake, but……
How much are you looking to spend? And how much are you looking to get?
Believe this or not.....

Stock 330 HP Mercruiser engines are; Cast pistons with peanut port heads with a CR of about 8.75:1, cast crank and a step above a truck cam.....

(I have three of them here..

.) Yours was either very early or someone did a presto chango for a Magnum.
But your performance estimate sounds typical for what you had.
There is nothing wrong with building a stocker with some good parts to get the boat wet. In all honesty, it takes more than 50 extra HP to actually have the Ass-O-Nometer tell any significant differences. Speed wise, 50 HP translates to approx. 2 MPH, so don't beat yourself up. Concentrate on tuning the entire package and you will get more enjoyment out of it.
Where I continually see guys and gals get themselves in to trouble is where they build a badass engine, but miss the setup. If properly tuned and with matching parts, a turn key 60+ MPH jet is pretty easy to achieve.
What I mean by "matching parts" is, you don't put a dual four tunnel ram with 600 CFM carbs on a low compression 454 and a stock grind cam, it just wont work well.

GT