The resistance to rotational forces will be provided by the taper, which is essentially a interference fit and does not need a key. However, taper fits rely on precise machining and can be broken free with a strong shock load.
As long as the nut is torqued and maintains the interference fit on the taper, no slippage should occur.
Note all the provisions for the taper to work effectively!
Given that we are looking at a steering application and it's on a boat (not optimal environmental conditions), adding a key way to the design adds a safety factor (see Mikes comment).
All being said, hubby is correct in stating that the key is not required mechanically, but not using it in this application isn't the proper solution.
Aligning the wheel with the drive can be done at the rack as Mike suggested or at the drive end where cable attaches to the steering arm.
If there isn't enough adjustment, this could be a sign that there are parts that need inspecting or replacement.