How do I host a server online?

You need to forward your ports if you’re using a router. Check out this guide by fansite Seal Hunter Pro.

Am I able to play with others online?

Yes, Seal Hunter is exclusively multiplayer, over either the Internet or a LAN. Note that this doesn’t mean that you won’t be able to start up a game all alone… just that there is a much heavier emphasis on playing together as a team.

How many am I able to play with?

You can play with up to three other players (i.e. 4 player games are the limit).

Are there new weapons?

Yes, there are, and more are planned for future updates.

Is this free?

Yes!

Does it work under Windows XP/Vista/7/8/10?

Yes, but you will need to run it with administrative rights.

Is this open source?

Maybe some day, but not right now. I’m still contemplating it.

Does this run under Linux/Mac?

While there are no Linux/Mac builds planned immediately from the get-go, players have reported the game working just fine under Wine. I wager it’ll be emulated just fine under Mac, too, in that event.