Ah...the stereo mod! It is always an interesting challenge. And much of what you describe is a big challenge. You actually are using your stereo inside a bathtub. And that is a poor arena for music. And your outside the tub setup can go several different directions. They make projecting speakers for the tower, that are called H.L.C.D. (Horn Loaded, Compression Driven speaker) that will ensure that a rider 80 feet behind the boat can hear and enjoy the music. Those are high dollar speakers and require a vast amount of power to run them, along with the batteries to support them. We love our Yamahas, but we have stator magneto charging systems that only produce 26amp recovery per hour on the twin engine boats. I don't know for sure, but suspect your more in the 13amp range. So adding a high draw sound system is kind of going to be a difficult thing on the single engine boats, not to mention room and weight issues.
Many guys add coaxials in the tower cans to get sound outside the bathtub, but don't care about playing music 80' behind the boat...and don't expect coaxials to play 80' behind the boat, they won't. But they will play well to the near range audience, say 20-30'. Without the HLCD speakers, your low end will suffer outside the boat as well, as subwoofers are not designed and don't get outside the bathtub well at all. But many of us have great sound systems that sound good running and while at anchor...and even sound good out on the swim deck and in the water behind the boat or at the beach, within 20 yards or so when at rest. The HU (head unit) is "ok" if it is working. The big issue is separation of zones IMO. But with an amp, you can isolate the zones with a line level volume control for the different areas of the boat, and a similar device for sub level control. Your Polk DB651 speakers are a high quality entry level speaker. While they are far above many entry level speakers, they are an entry level speaker. So by definition, they won't match many of the better stereos out there that have higher quality marine grade audio speakers. By match, I mean they won't deliver the amount, the quality, or have the longevity that some of the more expensive lines and models will have. If Polk is your line of choice, the MM651UM is a much more capable and durable speaker for the marine environment. I have used JL Audio and I am in my second boat now and doing that mod on it too. The MX650 needs about 60w to drive it efficiently, the MX770 needs about 70-100w to drive it, and the M770 needs about 150w to be happy. This is per speaker. You can drive the speaker you have, the DB651, with 50-60w, but what you get out of it won't be the same...but it will be a far cry from what you have now I am guessing.
You are limited to what you can do in a 19' boat, but there is a bunch you can do! And, you can do the horns if that is just what you want, you will just be limited on time. Because getting ballast tanks and 5 batteries or more, amps, and still have any room may be a challenge. But hey, we love a good challenge, right?!