I have worked with allot of oak, pine, birch and others. The process should be the same for teak.
You need to first of all, degrease the table top with a good cleaner. Remove all grime and grease and debris.
Secondly decide what color stain your going back with or maybe not any at all, you did not mention this. I have heard that teak is the finest left alone, do not know that for a fact.
Now, you need to sand down evenly and equally the entire surface to get rid of the water marks. Usually, water marks do not go down that deep. Maybe only 1/32 will do the job. I would definately use a belt sander to get down faster, BUT unless you are experienced with one, you might eat too much material and get an unequal sanding. So, you might settle in for a orbital sander or I would suggest sanding by hand with a medium grit sand paper until you get rid of the water marks, Once they are gone, switch to fine sand grit and as ALWAYS, sand in the direction of the grain, and NEVER go perpendicular. Once you have it nice and smooth, blow off the top and wipe clean with a very damp cotton cloth. The damp rag will cause the stock to rough up a bit, Now sand more until its smooth again, now blow off again and steel wool it. When its the stage you want, then consider applying a good quality sand sealer or conditioner. At this stage you would begin your application of stain or your sealer. Whatever you decide upon, you apply a light coat, let it dry, then buff with steel wool, wipe clean and apply another coat, keep this process until you get a good coating and nice sheen.
Come back with your intentions on either staining or just sealing.