What is the best why to get money on Endless ocean 2

Go to Valka Castle or Cortica River Upstream - where the Amazon River Dolphins are and use your Multi - Sensor to find treasure, and then return to Nineball Island and call Nancy on the radio and have your items which you found appraised. The items you want to be looking or are treasure chest because you will get a lot of money for them.
I also recommend you try multitasking. Try taking tourists on a guided tour and taking a picture of a photo request at the same time. You won't be able to fulfill salvage requests while going on a guided tour, but this is still time saving, and you earn more money from it.

Hope this helps. :)