Choosing a piston

Normally one chooses the pistons first, then has the block machined for them (with the specific application in mind to set the clearances appropriately).

Where exactly in the process are you?

With Pistons you have 3 basic types to choose from.
Cast Piston = same as the original piston in your boat.
Hypereutectic pistons = an improved version of the cast piston.
FYI: Mercury Marine has been buying Federal Mogul Hypereutectic pistons for their engines for many years. They work well in these mostly stock engines.
Forged Pistons = a very strong and forgiving piston but the highest priced piston.
Forged offer some extra protection inside the engine and can help avoid failure in extreme situations. If you are going to spin 7,000 RPM buy the forged.
If your block is a very high dollar and hard to replace block, buy forged.
In each piston type, Cast, Hyper or Forged they will have a few compression options you can choose from.
Typically a Marine block will be honed to leave a little extra Piston to Cylinder wall clearance to allow for extra thermal expansion and so you have no issues running hot pistons in your marine engine which will typically run at 140 degrees.
If the Automotive Piston is recommended to run at .0015 clearance most shops will add an additional .001 clearance for marine use for a total of .0025.
If you buy the Hyper pistons you can run them tighter, they don't expand as much.
Your machine shop can help decide what clearance is good in your block with the options you choose to run.