Try searching for Garmin or Lowrance for some light reading.
I use a 76CSX, but also have 2 or 3 boat mounted. A man can never have enough GPSes. LOL.
Any one will work out of the box to mark a spot and then come back to it. Any of them including the little dinky 99 dollar models. What you're paying for is screen size, screen resolution, expandability (like memory cards) and color or lack there of.
No, the maps are not necessary, but they sure are nice so you can see where you are in relation to another object like a channel or drop off.
First one I owned...well, actually still do, was a Garmin GPS12. It was medium priced for it's era and is now probably 8 or so year old and it would be just dandy to mark a spot and go back to it even now.
I have my opinions, and I'm sure you'll get some, but any of them will serve just fine for marking a spot and going back to it. The devil is in the details though, like which maps work with which units. Cable add ons, software to manage the way points, how many track and way points it will hold and on and on.
And I haven't even mentioned the new touch screens. Ha.
Oh, battery life. Yeah 10 hours is about norm. Course, you can get cigarette lighter adapters to avoid that (for a price of course).
Hope that helps some.
Wannabe...