The Hastings pages are good information, but unless you are comfortable doing what I consider major mechanical repairs I would direct you to a competent shop to carry out the operations. This involves removing the drive.
If you do plan on taking this on yourself here are some things to try:
First of all it is shifting into gear, both forward and reverse so I don't think the cable is out of adjustment.
Second, the ESA or Electronic Shift Assist may not be functioning properly. This is a little black box that causes the engine to skip, chug, gag or otherwise slow way down so that it is easier to shift. One reason it is harder to shift to reverse in the water is that the water adds resistance and load to the prop so that the dogs on the clutch are harder to release.
Third, your idle may be too high. The idle should be as slow as possible yet fast enough that the engine doesn't die when the ESA is engaged.
So, with that in mind, check the ESA module connections and grounds. There are aftermarket replacements now that are a little better than the old one as you can use them with electronic distributors. Check your idle speed. Also, when you shift, it's a one-two operation not a jam it into gear thing. Apply pressure on the shifter until you sense the engine gagging and then complete the shift.