Changing the cable isn't too difficult when the boat is out of the water as long as you pull the new cable though when you remove the old one. You should be able to tell if the cable is stiff by pulling it manually where the actuator is (there are some pins that you can remove for manual shifting in an emergency).
If the cable is stiff, don't be tempted to continue to use the boat as you'll knacker the actuator and that's a £1000 part.
mechanic replaced the cable for me recently
it needs one inside and another outside to get the cable through
if the access is poor inside (def the case in my boat) you might want to pay someone else to do it
not difficult technically, can be a tight fit