The correct statements among the given options are:

I. Tides are of great help in navigation and fishing.

II. High tide enables big ships to enter or leave the harbor safely.

IV. Kandla and Diamond Harbour are tidal ports.

Therefore, the correct answer is: I, II, and IV.

Let`s explain each statement:

I. Tides are of great help in navigation and fishing:

This statement is correct. Tides play a crucial role in navigation as they can be used to determine the water depth and currents, which aid in safe passage for ships and boats. Tides also impact fishing by influencing the movement and behavior of marine species.

II. High tide enables big ships to enter or leave the harbor safely:

This statement is correct. During high tide, the water level in the harbor rises, providing sufficient depth for large ships to navigate safely. High tide allows ships to enter or leave the harbor without encountering shallow areas or obstacles.

III. Tide prevents siltation in the harbors:

This statement is incorrect. Tides do not prevent siltation in harbors. In fact, tides can contribute to the movement of sediments and siltation in some cases. Other factors such as dredging, sediment management, and engineering interventions are used to prevent siltation in harbors.

IV. Kandla and Diamond Harbour are tidal ports:

This statement is correct. Kandla Port in Gujarat and Diamond Harbour in West Bengal are examples of tidal ports. Tidal ports are located in areas where tides have a significant influence on water levels, allowing ships to enter or exit the port based on tidal conditions.

Therefore, the correct statements are I, II, and IV.