What debris? Everything was sparkly clear across the entire horizon in every single shot we saw. The "heroes" also didn't seem to have any trouble finding Strand, and considering he was pretty much just holding on to debris, so if the ocean was littered with it, it'd be a pretty hard pill to swallow that they'd find him as easily as they did.Depending on the sensitivity of the radar, how much debris is in the water (a lot, apparently), the height of the waves, and how much chop the yacht's kicking up in their wake, a tiny Zodiac like that probably could just blend into the background, especially if its mother vessel remained out of detection range. They probably used debris fields for cover until they got close, and then snuck up from astern using the yacht's wake to conceal their approach.
I had no idea satellite phones worked on the same frequencies as radios.As far as how they found the yacht, they were dialed in to their radio frequency since Alicia foolishly started an apocapodcast with her new pirate BFF, so when Strand started contacting Luis to set up the rendezvous, they might have been able to triangulate their signal again. Or they have a more powerful radar on a smaller boat which allows them to track the yacht while remaining out of range of the yacht's radar. If they took a small Coast Guard patrol/SAR boat or a Sheriff's boat that would be ideal.
And if they had a boat that powerful, it's kind of iffy why they'd be interested in such a small yacht anyway, especially one clearly already being manned. Again, there's plenty of fish in the sea as far as that goes. Chasing one relatively tiny boat across the region is... well, ridiculous is the best word I can think of.
Even if they didn't have the technical expertise to start one up that they found immediately, they could always just latch it on and tow it until they figured out a way to do so. Hell, just nudge it far enough into the sea, make sure it can't be boarded by mindless zombies who can't figure out ladders or steps, and you still have all the time in the world to figure it out. You even have an escort gunship (presumably; still assuming they're the same ones who inexplicably destroyed that other ship) with bad-ass super radar onboard to make sure other would-be pirates were seen and dealt well before they become a problem, all the while you have people on the yacht working on getting it going.