Always consider the appearance you are trying to achieve, any environmental factors and boat use when deciding which sealer to use; some do not change the appearance of the teak if you like a natural look. A few popular sealers are Semco and a new product call Triton by Ditec. Teak oil products are also an option, although some will promote the growth of mildew — probably not something you want to use if the boat sits for a long time without regular maintenance. In sport boat applications, oils are usually only used on covering boards and deck trim, and will definitely change the look of the wood. Again, consult a teak professional or a captain with a boat that has the look you are going for to be sure the product will work for you before applying it.