Depends on how often you'll be having tea in the cabin. Room service is not the fastest thing in the world if you find yourself craving a tea "NOW!" (Although ChucksOk probably gets immediate service on the Seabourn ships as opposed to the ones we commoners travel on. ) But aside from room service, there is always hot water dispensed from the beverage "islands" in the buffet area.
Nothing says you can't bring your own travel kettle, but keep in mind there is typically only the one power outlet in the cabin. If you don't wish to have to pack the kettle, Chucks suggestion of room service would do.
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!
You can also order a pot of hot water from room service, if you bring your own tea as my B-in-law did on our last cruise. Tipping is still appreciated. If you know when you'll be up, you can leave your order on your door handle the night before for delivery at a specified time.
On some cruise lines when you get room service they want you to sign a slip. They say that it is for accounting and service evaluation. In reality it is just a way to say that you should tip them because there is a line close to where you sign that says TIP. That is the only line they are really concerned about.
Yes there is a $10 gratuity recommended charge to your account everyday, but that is for normal service. Anything beyond that justifies more.
Is it worth going up to the buffet and back for a dollar? Or think about the employee that has to fill the pitcher and walk the length of the ship and a few decks for hopefully a dollar.
To me the biggest rip is the guy at the luggage check in that puts your bags into a container and DEMANDS a dollar or more per bag.