Wow you're getting a lot of advice from people who've never used a sonar...
I have a Garmin
740s with a 600-watt shoot-through transducer.
If you have a cored hull
, you'll have to pour some epoxy
to avoid new holes. It's worth it - external transducers are a pain. (Worth it if you need them, but you don't.) A transom mount (the "three screws") might be OK if you have a place for it, but I suspect if you get this thing you'll come to enjoy using it, and that a transom mount won't work for much more than puttering around an anchorage. (Transducers are VERY sensitive to bubbles of any kind.) If you're really OK with nothing but puttering around an anchorage, a stand-alone unit ($100) probably makes the most sense.
A patchy bottom will look like patches, and seagrass will look a lot like seagrass on a decent sonar. Finding those things are WHY people buy sonar units, not some fatal error to operation.
My transducer DOES have limited power - if you're anchoring
in ~100 feet or more (and presumably have NO idea what might be at the bottom of the big rocky cliff), you might want to go to 1000w. I'm not sure where I lose resolution, but at ~40' I can usually see a couple feet into mud, and lose the bottom altogether somewhere around 2000'. I suspect you can about always see where your anchor will end up.
(Since someone asked, the 1KW have more "antenna" in there - they have more power AND more resolution. If you're fishing
daytime swordfish or grouper in deep water
you NEED this - otherwise, maybe not so much. I'd love one, I just don't want the big hole, in my boat OR my checkbook....)