Dishwasher + Instant Hot Installation
The best solution is to actually change the single gang box to a double gang box and install a second unswitched duplex outlet.  This is not actually difficult for an experienced electrician, the DIY skill level is moderate.  The total draw is only near max, if both dishwasher is on dry and hot water gets used, but insta-hots actually only run for a very short cycle if used.  It shouldn’t trip if there’s nothing else on the circuit.

That said, I used this: https://www.homedepot.com/p/GE-6-Outlet-Grounded-In-Wall-Adapter-White-54947/203742151 or equivalent... specifically a model tho with a center screw, that’s critical for this application.  If it’s the typical garbage disposal/dishwasher under sink plug that is a half switched duplex outlet, then the other two plugs on the switched row of the adapter are pretty useless.  Regarding 15 vs 20 amp ratings, I think you won’t find 20 because it’s a NEMA 5-15 pin configuration.  

(Footnote: I plan to change this when we remodel the kitchen and the cabinets are out to the 2-gang 2 duplex outlets described at the beginning, due to the location of the stud and the side they chose to nail the original to—mine was not the simple case of make a 1 gang into a 2 gang, mine actually has to change to the other side of the stud as well, I opted to use the adapter and have not had any problem.)