The alarm could sound for a high number of things going wrong and about the only way to pinpoint what it may be is to have the engine scanned with a marine style scan tool of a few references of types available.
Your alarm is sounding a hard alarm at about 4200 RPM which is considered a critical issue to go into guardian mode like that.
Unfortunately, everything is a guess until scanned right down to depending on what design (number of wires and how connected up) the trim sending unit/s are for the trim gauge (which I can see in the video is broken/stuck all the way up) and could possibly sound the alarm to where it thinks you are way over trimmed at high RPM and lower the RPM until idle then do it again at 4200 until fixed... But again, just a guess not knowing you setup and there are many other things that could do the same exact thing especially on your engine model and the year of the boat as that is the year things really started to evolve technology wise for complexity.
Is your engine digital throttle shift or still mechanical throttle and shift (cable vs wires)?
The water pressure switch is depended on your engine models serail number as some may be mounted on the power steering oil cooler near the back starboard side or possibly moved back down to the sea water pump impeller housing item number 12 seen here,
https://www.mercruiserparts.com/bam/subassemblydetail/43086244/28188353
If you are more of a do it yourselfer vs finding a marine service shop/mobile marine technician then there a couple of offers on Ebay to where you can rent a diacom kit to plug into a laptop and read live data and fault codes to see where you need to head for repairs that or if the budget allows of high dollars you could buy a diacom kit or hand held scan tool by buying either direct from rinda technologies or sometimes seen on Ebay too depending desired scan tool model you'd prefer...They tend to start at the $500.00 + range.... Renting one for 7 days is about $50.00.
▪ DIACOM Marine PC Software
▪ TechMate Pro Scan Tool
▪ Diagnostic Adapters
▪ CodeMate Code Reader
▪ Other Marine Products
▪ Web Store / Online Ordering
The Official Site of Rinda Technologies, Inc. Designing innovative engine diagnostic tools and software for over 20 Years.
rinda.com
https://www.ebay.com/sch/i.html?_nk...2334524.m570.l1313&_odkw=rinda+tech&_osacat=0
If you need pro help and looking for mobile then depending on your location sometimes under craigslist under services/marine you might be able to find someone to help out on the water, etc..
