The easiest way to get there is taking Chao Phraya Express Boat service. You have 2 options. First is getting off the boat at Tha Chang (close to the Grand Palace) and then head north on Maharaj street and walk for about 5 minutes. Another way is taking a boat to Wang Lang and take the ferry cross the river to Tha Wat Mahathat which is adjacent to Tha Maharaj.