I'm also an independent game designer. I can do most everything on the design end except make art assets - models and promotional images. My main talents lie in programming and writing.
I spent the better part of a year devoting myself to learning how to code for multiplayer, and it really is a pain in the rear. It has to be something you plan from the beginning, and even then, you have to know EXACTLY how the whole system will come together before you write it; alternatively, you have to go back through your entire set of code in all of the scripts used just to rewrite it to include multiplayer support. By then, though, the game should already still work, so you still have to test it again, thoroughly, to make sure you didn't miss anything in multiplayer, or you need to have several people proficient in multiplayer coding to proofread all of the code too, to double- and triple-check that nothing was missed. And you'll have to debug it, too, which is even worse because, again, the code already works. So you have to find where it works but doesn't.
Every input needs checks for singleplayer, client, and host or server. Any inventories need to be known by a server, as well as any changes and when. Non-authoritative p2p is prone to lag and needs predictive ai to represent other players, to better understand where they are and what they're doing; ai in p2p needs some form of synchronization, so even the shark existing complicates matters. Checks need added to update positions and changes that may have been missed. All connected players need managed so the game knows who is allowed to play, and who gets the data. Everything needs to check if in multiplayer to disable communication attempts so it doesn't waste resources in singleplayer. It's like programming your game layered overtop of your game, and requires points in the singleplayer code to connect to the multiplayer code; you can't just add more code to it and hope that it works, the way you would add new content.
It takes way more effort than it's worth for a small group. It's not impossible, but it's definitely "super difficult" without a team devoted just to it. If they aren't planning multiplayer now, it will be even less appealing when there's more content, and they probably never will. That's the sad truth of it. Knowing how to code is different from coding for multiplayer, and the task is just too ambitious without several experts on the matter.
