Tackle
A combination of blocks and line used to increase mechanical advantage.

Thwart
A seat or brace running laterally across a boat; also, a rower's seat extending across the boat.

Thwartships
At right angles to the centerline of the boat. 

Tide
The periodic rise and fall of water level in the oceans.

Tiller
A bar or handle for turning a boat's rudder or an outboard motor.

Topsides
The sides of a vessel between the waterline and the deck; sometimes referring to onto or above the deck.

Topgallant
Relating to the part next above the topmast and below the royal mast. 

Topsail
The sail above the lowermost sail on a square-rigged ship; also, the sail set above and sometimes on the gaff in a fore-and-aft rigged ship.

Transom
The stern cross-section of a square sterned boat.

Trim
Fore and aft balance of a boat.

Trimaran
A boat with three hulls.

Tripline
A line fast to the crown of an anchor by means of which it can be hauled out when dug too deeply or fouled; a similar line used on a sea anchor to bring it aboard.

True North Pole
The north end of the earth's axis and also called North Geographic Pole. The direction indicated by 000° (or 360°) on the true compass rose.

True Wind
The actual direction from which the wind is blowing.

Turnbuckle
A threaded, adjustable rigging fitting, used for stays, lifelines, and sometimes other rigging.

Underway
Vessel in motion, i.e., when not moored, at anchor, or aground.