On land, a Trade Route can reach destinations within a range of 15 tiles. Once the player researches the technology that allows embarkation, Traders can also move over water. On water, the range is 30 tiles. If you have a Trading Post in the city, the Trader "refuels" its allowance for the new travel domain, and can move further. Otherwise, it must stop at that city.
Changing between land and water can only be done at a City Center or Harbor district, and the allowance does not carry over.