Submarine swaps are more of an on-ramp to the Lightning Network. It allows someone to receive a payment on the Lightning Network without the payer using the Lightning Network. I believe it also works in the opposite direction too, i.e. the payer is using the Lightning Network but has to pay someone that doesn't use the Lightning Network.
Interesting. Compared to LN and atomic swaps, is it going to take more time to be live? I've seen a video about it on /r/Bitcoin a couple of weeks ago and the only repository I found is this one: https://github.com/submarineswaps/swaps-service so It's hard to tell how active the development is.
