Probably a bad idea. Rocket launches tend to have exclusion zones for sea traffic and they will scrub the launch if some boat wanders into the area, and speaking as someone who likes to watch rocket launches on stream, I’m confident in saying that lots of people will be mad at you for doing this.
Edit: Oh, you said Matamoros, which is in Mexico and presumably out of the FAA's jurisdiction. Everyday Astronaut seems to think Matamoros is one of the more dangerous parts of Mexico[1] but I'm not one to talk anyone out of adventure travel. Just be wise to what you're getting yourself into.[2]
Looks like you can get like half a mile north or south (based off some crude eyeballing) & be out of the easterly-directed exclusion zone. I have no clue how that compares to where people can be on land to watch, how far up rt 4 people are allowed. It seems like it might be a good option.
Also the launch should pivot & head out more your-a-way too, which could be cool.
The green areas are the closest you can get by boat. Which is basically just as close as viewing from South Padre Island. You might have a slightly less obstructed or crowded view from a boat versus parking and standing with everyone else on South Padre.
Boats in the exclusion zone can and have caused launches to scrub. I can't speak as to whether you will face legal consequences, but there may not be a launch to watch if you violate the safety perimeter.
I had a similar thought about watching it crash down into the Pacific off of Hawaii. If it makes it that far, they plan to sink it in a naval graveyard about 50 miles north of the island. Watching it attempt to soft landing and then sink below the waves would be remarkable
This second stage is going to slam into the water at terminal velocity (per FAA filing). No mock landing flip and burn. The two after will break up on re-entry (no "wings", people suspect they are for payload or propellant transfer tests).
The booster, however, will do a mock landing over the water a ways out from the launch facilities (40 miles I think it was).
Edit: Oh, you said Matamoros, which is in Mexico and presumably out of the FAA's jurisdiction. Everyday Astronaut seems to think Matamoros is one of the more dangerous parts of Mexico[1] but I'm not one to talk anyone out of adventure travel. Just be wise to what you're getting yourself into.[2]
[1] https://everydayastronaut.com/how-to-visit-starbase [2] https://www.reuters.com/world/us/mexican-cartel-says-sorry-a...