Zoran, You may be over thinking this but good points there to help justify consideration for a manual switch.
You have a 60 Hp engine, so not a huge boat. Are you a bait fisho or a offshore fisho? Trolling cover some distant? Downrigging?
Can you calculate and measure your power requirements (what electrical devices. what it consumes, how long run for?) every day on the water could be different typically sounder ,lights, livie pump goes on all day and possibilty electric reels. If you are a bait fisher anchor up and engine off. Manual switch will be fine but I would still get an auto charge.
If you are an offshore fisho cover alot of distance , have electronics running all the time and not sure what everything uses and past age 40 (memory loss which battery did i use last time?). Then definitely get a VSR kit.
For peace of mine ..grab an add a 2nd battery kit. When you add a 2nd battery in a boat or a 4wd it is best pratice is to put in a VSR (Relay) and it comes with the kit with a switch, to close the circuit to charge the 2nd battery. With a 4wd you could always walk home.
A VSR it is just a relay that closes the circuit . VSR , a Voltage sensitive relay it will closes when it senses 13.6V.It isnt that hard. When you get the boat with single battery, it is all wired into one circuit to one battery.Just leave the engine and bilge pump 's circuit on the first battery. Add a 2nd battery and put all the accessories onto a new circuit that connects to 2nd battery so now you have isolation. Add the VSR to bridge the two for charging.
BEP and Blueseas makes kits for Marine.
Check them out, the Blueseas i think is better as it has one switch that does it all unlike the BEPs multiple switches.