This type of freight would be moved by an "LTL carrier". UPS purchased Overnite a few years back, Fedex purchased Viking, so both now do LTL, but the operations are different and separate from the package service that we all know. UPS Freight is also fairly expensive compared to other carriers. You need to find a third-party freight broker and you should be seeing rates as described below. This is really true for anyone moving an outboard, I did the rating at 400 pound but that's still hitting minimum charges, even at 600 pounds the base rate only goes up by 10 bucks.
If you pick up the freight at the destination terminal it will substantially reduce your shipping expense. You are probably paying an additional $50-$75 for delivery liftgate and that amount again for residential delivery. Your base transportation rate should be around $100-$150 , liftgate and residential delivery will add anywhere between $50-$150 on that. The numbers depend on the broker's markup, I work for a shipping company and I just pulled up rates for one of our third-party brokers (the rate we would extend to them) and it'd cost $161.47 including res del and liftgate or $107.97 without. Shipping is dirt cheap, our cost on that freight move is $43.72, add another $25 for liftgate and $25 for pallet jack. $44 bucks brought a motor from Michigan to my front door in South Florida (the carrier waived the accessorial charges). No sales pitch here, we don't do individual moves, but at least you have some behind-the-scenes numbers to work from.
Your outboard is "class 85" for the purposes of rate calculation. It should be fine strapped securely to a pallet. Palletized freight is not typically stacked so its not nearly as damage-prone as packages.
Hope this helps,
-E