The pukka NEP 2 is an expensive £235 or thereabouts. Cant imagine that B & G marine networks are the only system to require an ethernet expansion port, so is there an alternative?
I have a few tp-link SF1008D 8 port 100 Mbps switches on board... I choose that model because checking out the power regulator in it, it seemed happy with anything from about 10v to 19v supply and being 100Mbps looked like it used less power than faster switches.
Sometimes I power up a small tplink WiFi router as well. They're very light weight and small which is a bonus.
As to connectivity, I've just chopped a few Lowrance Ethernet cables in half and crimped on standard rj45 plugs.
As to longevity, they've been on board for at least five or six years now...