Dry Cell batteries

Can somebody tell me the good points and bad points of using a dry cell battery as a 2nd battery. I understand you cannot use a standard car battery charger for these - instead a specific dry cell charger is required. I am assuming the dry cell battery would have no adverse affects being charged off the boat alternator when the engine is running?
My plan is to add a 2 battery perko switch and use the dry cell as the "get out of jail battery" if my primary battery goes flat. If I can get away without relays and combiners I would rather do this. Thoughts and comments appreciated. Cheers.

A true dry cell battery has to be charged in a very specific way otherwise you slowly destroy the battery. Just go with a AGM battery they are less finicky and cheaper.

I can get the dry cell battery for free hence was hoping I could use this in my boat. Will the dry cell charge from my alternator or would I have to have to hook it up to a mains powered charger when the boat is out the water?

Yes a drycell will charge from your alternator but it might not charge it effiecently and cause premature battery failure. If your getting them for free I guess it wouldnt matter cuz you could just replace it if it fails. If you dont want to destroy the batteries it would not be a bad idea to keep it off the alt and get a good charger that handles drycells to charge it independently.