There are no waterways in Nepal for inland navigation. The only practical seaport for goods bound for Nepal is Kolkata, India.

Nepal and India have carried out studies on the possibility of having navigational use of the Gandak River system. Along the Ganga-Bhagirathi-Hooghly river system, National Waterway 1 (NW 1) of India Gandak river meets at Hajipur (marked with green circle) and Koshi river meets at Kursela (marked with blue circle). The two rivers from Hajipur and from Kursela to Nepal are currently not navigable and require dredging. India started dredging the 300km river section from Hajipur to Nepal in 2020, to make the river navigable for 1,000-2,000 MT vessels. The confluences of these rivers with the Ganga could potentially link Nepal to the Port of Kolkota via river. However, cargo transport from Kolkota port by river would need to compete with the cost and speed of transport by rail and road. The Nepal Shipping Office was established under the Ministry of Physical Infrastructure and Transport on 14th February 2019 to oversee the progress on Nepal-India Transit Treaty to use three inland- waterways (Kolkata-Kalughat-Raxaul; Kolkata-Sahebgunj-Biratnagar and Kolkata-Varanasi-Raxaul routes).

(Sources: Are Nepali waters navigable?; http://www.indianetzone.com/29/gandak_indian_river.htm)

[image from Water Resources Information System of India]