As has been suggested, the first you check is if you have enough voltage to crank the engine. Say 13+volts...
If the voltage is there and it seems to you that the starter is trying to turn the motor but is failing to do so, you may want to use the de-compression technique - If your engine has decompression levers, switch them both to de-compress position. That may allow the starter to turn the motor with its partial juice. Once the motor is turning, switch the de-compression levers one by one. Once you switch one the engine will start as the compression occurs (it may be rocky) and once it starts then you add the second cylinder. This technique was sometimes used in situations like these. Of course this can be done only if you have de-compression levers.
However, if you have good voltage and still the starter does not crank (you check wiring is all ok but you get nothing), you may want to check the fuse that is there on some diesels inline with the connection from the start switch (push button switch) to the starter solenoid. In some boats this fuse (usually 20A) is located in a very inconspicuous place. I have advised people to lengthen the wires and move the fuse to a more accessible and visible place.
Just some thoughts.