I could argue to not to try to do all, but do the stuff that you can do better and will save you time and money.
Simple things like changing oil/filters/batteries, etc., are often done faster than calling someone to do it. Even the big batteries. I've often jumped in to help a buddy with them and can get two installed in no time.
I really like to hire out what I call grunt work. Work that doesn't necessarily take a lot of skill and you can get people to do it cheap... like cleaning the boat, detailing, mowing the yard, etc.
I like to do skilled stuff that I've learned to do well that's expensive to farm out and often have to wait for services to be done. Especially things that require taking the boat to someone. I've got things set up where I can pull the drives if needed, able to replace a sea cock or thru hull item, or a bow thruster. All of those get very time consuming when taken to the boat yard.
I will hire someone to replace starters, as they are a bitch to get to, below and at the rear of the engine, but have done it.
Now, I could argue that if one does not do their own maintenance, they should take the time to know how, or at least the basics to be sure things are done to their satisfaction.
Usually the mechanics are fine, but seen some cases where neither the mechanic or boat owner knew what was going on and ended up ugly.