Do what I did and get yourself a GPS speedo.... no more water, sensors or wiring to the harness. $55 on eBay and you grab some power from the ignition. I did it to mine and it works great.... so easy AND cheaper !
Can you confirm your year and model? What's a Seadeck ? If you mean Sundeck, then this should give you ALL the info you need. Good Luck !
https://s3.amazonaws.com/srweb-pvt-prod-us-east-1-294962523550/static/parts-manuals/2009_280-Sundeck_280-SD-Parts-Manual.pdf
Might you have a pic of this area? I can't find anything on my engine where my speedo hose would come into a barb on my engine. Do you know where on the harness it plugs into? I've repowered my 05 SR Sundeck 240 from a 5L to a 350 MAG and I imagine nothing was done to remedy this on the new...