Yes, tip the room service. But tip cash as service is given.
Coffee and or Hot Water for Tea
$1 - $2 depending on speed of service.
Sandwiches tip more in the range of $3 to $5 for
speed of service, and order size.
I do want to point out that Cabins on a Ship are not like hotel rooms. You can bring a hairdryer, but a kettle type thing a no-no. This is a Ship, and they don't want any fire concerns. I drink Tea too and either my Wife or Buddy orders "Coffee for 1 and Tea for 1" from room service and tips a buck or 2. You can write this on the ticket if you don't have any cash at hand.
You will be fine, they have all this available, but the tip is for them bringing it to you. You have more Class than some by asking!