Quoting a post I put in the Whale’s Treasure survey thread…
If people are wondering about the extra battles it goes like this…
Archeloth = 10 Finns instantly (refreshes every 2 days)
Seaviper = 6 Finns after lvl 3 difficulty water battle (refreshes every 1 day)
Tidalwhale = 3 Finns after lvl 2 difficulty water battle (refreshes every 2 hours)
Seabite = 1 Finn after lvl 1 difficulty water battle (refreshes every 10 mins)
After a battle becomes available it then becomes a % chance of appearing on the map each time you change screen to it (not from the monsters menu).The Archeloth has an incredibly low % chance of appearing so you may be refreshing the map for a few minutes before you see it.
As you can see, the Archeloth and Seaviper are perfectly fine to farm and give lots of Finns. I liked farming the Tidalwhale (because I love to grind) but the Seabite was a ridiculous 10 mins cooldown and only gave 1 Finn every time.