I think its important to note that passive radar isn't always the best idea.
I'd like to think that I'm fairly proficient at this game, so I'll throw in some things I've learned.
1) I know it's been said before, but cover is important. What most people don't realize is that the type of cover is equally important. You need to know where the missiles are coming from in order to properly use cover, and you need to know what cover your Mech can fit behind. A small column of rock might be enough to hide a Raven, but a Thanatos just looks stupid trying to hide behind it. Also, the same pillar might be good against missiles coming from in front of you, but won't do shit if the boater is on your sides.
2) Passive radar can make you harder to find, but most boats have some sort of active radar probe. Passive doesn't mean invisible, and if you run around in the open with passive, your still gonna get spotted and destroyed without even knowing what hit you. Active is important to use, but only in short pulses. Again, you need to know where missiles are coming from in order to take cover against them.
3)Move as a group. Everyone loves to pick on that one guy by himself, don't be that guy. Keep in mind that missiles need a lock to fire, and with several mechs moving quickly through cover, boaters will often panic and not be able to get a good lock.
4) This kinda goes with number three. Work as a team: if you get hit by lrms, call it out to your teammates. Try to get grids and maybe even the type of boat. Roll out Missile boat counters like ELRM's, Long Toms, and Aeros. Do it yourself if you have to, don't just wait for someone to solve the problem for you.
Also, note that on most boats, the launchers themselves are very vulnerable. ERPPC's, Gauss, and AC's can hit beyond their stated range, and the ERLBL is only 200m short and perfectly accurate. Take out the launchers on that catapult and what's it gonna do? Blow the leg of that vulture and render it relatively useless, 4 LRM20's don't work to well when you can't see shit cap'n.