Good question. At first I'd say not a good idea. I recall
uses a twisted pair inside a shielded cable for their SeaTalk
buss. Stock Cat 5 cable is twisted pairs but NOT shielded. Although you can get shielded Cat 5. But if you're going to that much trouble just get the twisted pair/shield cable in the first place. I think they are both 22 gauge if my memory serves me (but that's not guaranteed!). Anyway, I'd go with the shielded cable on a boat
because I'm pretty sure your Raymarine ins and outs will call for a +signal, a -signal, and a shield. Good luck. BTW, I'm about to interface my Raymarine SPX-5 autopilot
with my Garmin
and also all my "wireless" TackTick sailing instruments. I think I'M the one going to need the luck!