Hi rsudh557, welcome to the forums,
I will do my best to answer your questions below, and if you have any further questions, or if I missed anything, please let me know.
When playing Stranded Deep, there is 1 Single Player and 1 Multiplayer Game per save slot. Both of these games use the same map layout, however progress on one, will not affect progress on the other. For example crafting a building in Single Player will not cause the same building to appear in Multiplayer.
With the layout being the same for the slot, the cartographer will display the locations of the bosses for both game types - and to answer your other question, yes the 3 bosses are available in Multiplayer too.
While they are two separate games and gameplay within one does not affect the other, changes in the cartographer in the menu will affect both, so if you change the seed number to reset the map, or replace an island on the map, it will affect both games.
There is an island square coloured red to indicate where you are in the map, and this will prevent you accidentally changing the island you’re on if swapping custom islands in and out. However, please note that only the player location in the Single Player game affects the cartographer, it will not show your location for the multiplayer game - so please use caution if changing islands for your multiplayer game to ensure you’re swapping out the correct ones.
You may also wish to know that in the main menu, days survived are listed for single player only, if you wish to know your days survived for Multiplayer, you can see them in the pause menu while in the multiplayer game.
Regarding struggling to locate sharks, did you set the settings to passive from the beginning? If you started the game this way, it may be possible that it prevented them from spawning entirely. Have you seen any other creatures such as snakes, large boars or large crabs when set to passive?
Lastly, as you mentioned fighting the bosses, please note that there is currently an issue in the game that prevents the crafting menu from unlocking the “Story” craftables after a boss is defeated. The team are testing the fix and aim to have a hotfix released for this issue asap. You may wish to focus on leveling up and crafting while waiting for this fix to be released.
If you’ve any other questions, please do not hesitate to ask and I’ll answer them as best I can.
Thank you