Hi, I run an online fishing show in New Zealand (www.jetskifishingchannel.co.nz or www.facebook.com/jetskifishingchannel ), so do a lot of fishing on my Seadoo GTX 155. I use a Lowrance HDS5. Brilliant unit good to 5000 feet. I have personally had it in 1000 feet and the signal is as clear as in 30 feet.
my advice, to get it right the first time.... your best bet is to contact "fishton" on www.fishtec.co.za
he'll be able to identify and supply you with exactly what you need, at an affordable price. if you search his name on sealine you could pm him as well.
Iv'e also looked at the Lowrance HD..i'm concerned the unit will be too heavy for surf launch..can you send me a picture of how and where you mounted it please.
The best Lowarance you can afford. Elite is fine. HDS series tops but pricy for most of us. Garmin makes a great GPS's for road, hiking, flying, etc. but have never been famous for their "fish finding" technology. I think Eagle is also made by Lowrance and slightly cheaper.
I am sure they are all good. I have the Lowrance Elite 5 with colour screen. I can't fault it. Mounted onto my console above the Speedometer etc . I have flipped the ski upside down in the surf and emerged with the Lowrance still intact and working perfectly. Sounds crazy to launch with it mounted although, going through a river mouth with rocks it's nice to have the rocks plotted and know the depth of water with respect to sand banks etc when launching.