If the engine is not worn out, reluctant cold starts may be a defect in the glow
plug system, or low
compression caused by stuck piston rings. Diesels that are not maintained with either frequent
oil changes or good bypass filtration are prone to stuck rings, especially if not run hard regularly.
Diesels that are idled for long periods or not run under significant load can also develop glazing on the cylinder bores which may also contribute to low
compression. As compression is what causes the engine to fire, low compression means hard starting.
If addressing those potential problems does not resolve the issue your engine is probably in need of a rebore and new rings and pistons.
Your first step, after a careful look-over and a compression test, should be to change the
oil and then warm the engine up and take your
boat out for a near-full throttle run of several hours at least, keeping a close eye on engine temperature while you do so. If your rings are moderately "sticky" and the bores moderately glazed, that may be enough to restore better compression. A solvent such as "Sea Foam" may help speed that process.
Of course if
maintenance and use fits the previous description, then the "engine workout" process has to be repeated regularly.
If those steps don't
work, pull the injectors and have them tested, and check the timing of the injection
pump.
Maybe start with a compression test first of all.