Mine is very tight, and needs heating and hammering off there.
After cleaning it up, it tends to go on a bit easier.
The manufacturer is anxious that it not be loose.
A sliding fit, by definition must have a clearance in there, and clearances fret and enlarge with time. You don't see sliding fits on prop couplings often. You really do not want it to slide on there easily. That's not the idea. You really don't want any "rock" in there.
By contrast, an interference fit needing heat and/or hammering and/or pressing to get it on there does not have a clearance when cold, and so tends to stay on, but it needs force (or heat) to get it on there. It won't slide on cold, but when you get it on there, it does not rock, and that is what the designers want!!!.
Still use the key and the set screws though... they still help a lot for security. The key stops it turning, and the lock screws stop it backing off. Check them occasionally too.
Bearings on a motorcycle crankshaft are like that. They need to be pressed on (you can't get a press in that bilge right now!). Once pressed on, they don't move, and they won't rock off.
It is a pain when you try to get the coupling off 10 years later, but it is necessary for it to be tight like that.
When you fit the new one, put the new coupling in the oven
(or blow-torch it), and lay an ice bag over the end of the shaft (and key) for an hour or so. Spray some WD40 on the end of the shaft quickly to displace the moisture or it will corrode between the surfaces when you get it on!!!, and hammer the hot coupling on there, with caution. The blow torch will make a big difference, the ice bag not much difference (but it helps).
You could even use some dry ice (CO2 solid) on the shaft, but it's very cold indeed, perhaps -110 degF, and I would be wary of hammering it much.
Wear gloves for hot and cold.
I know little about split couplings, but they probably go on easily, then need to be tighened to gain the interference they need. Keep checking the set screws though... it looks like death if they back out.