I would agree with Scott. Look for a good mild marine cam. The 307 engine isn't going to produce 300 hp with just a cam so just look at a mild upgrade unless you invest in a new intake and carb upgrade as a bare minimum. Even then, a major increase in power will be tough with that engine. See if you can find the specs on the original cam and step up a bit from there. Some machine shops can take the old cam and measure it for you. Auto cams tend to raise the rpm that the engine makes power in and the max HP will be higher than what you can safely rev the engine to for long periods of time. Check the max rpm rating for your engine and try and stay within that rpm. Once you start getting into performance cams they are by and large designed for drag racing and thus short bursts of power. Not WOT for long periods of time. Look for a cam that makes its power up until about 4400 or 4800. Depending on your manufacturers suggestions. I would also replace the lifters when installing a new cam. They will most likely be close to junk and they break into the cam much like races do to bearings. You will have to take them out to install a new cam anyway. Buy assembly lube and slather the cam and bottom of the lifters real well. I would also suggest submerging the new lifters in oil at least overnight to help pump them up if they are hydraulic. I would stick with hydraulic. Mechanical lifters need to be adjusted every so often and I can't imagine that being a fun experience in a boat.
Other things to consider when installing a new cam: check all pushrods for straightness. You can roll them on a flat table or counter top to check. Also, check the bottoms of the rocker arm nuts. They often are cracked and will not hold the seat when torqued. It is worth buying a new set. You will also need a new timing chain. Also, get yourself some long bolts that will thread into the front of the cam. Long enough for you to use as a handle for installing the cam. Be carful to not nick the cam bearings when installing and removing the cams. These bolts will help you avoid doing so. Good luck
