Re: mercrusier 2.5 need help
Start with the basics...
How old is the battery? If it's new, make sure it has a full charge.
If it's old, has it gone dead in the past? If so, and especially if it was dead for a long period of time, chances are it's in rough shape. Would be worth bringing it to any number of parts houses that offer free battery testing to see if it's still usable or a core return. How do the battery cables and connections look (both at the battery and engine)? Regardless they should be taken apart, thoroughly cleaned, and inspected. Any signs of corrosion, or acid infiltration in the cables from the battery warrants a new cable.
Have you tried turning the engine over with a socket and ratchet on the crank bolt with the plugs out? It should turn fairly easily. If not, remove the outdrive and repeat. If it gets better with the drive off you may have issues in the drive. If it has no difference, you may have issues in the engine. If the engine turns over by hand easily with the drive installed, AND, you are certain that the battery and its cables are in good shape (this is after you check the battery and cables), your starter might need a rebuild.