[Edit]: A disclaimer: I am by no means an expert, I'm only sharing what little I know, and my understanding of the situation.
Very difficult, whether on land, or in the waters.
Blockades take a very long time to set up, and you have to deal with the enemy impeding your efforts.
In the case of an Indo-Pak war, can India blockade Pakistan? Yes, but it would take a very long time, and generally wouldn't be worth it.
Most of the fighting would happen in the plains of Punjab, and a lot of the supply lines would likely be connected to China, and maaayyybe Iran (if Iran sides with Pakistan, or even if it decides to remain neutral). In which case, India would have to deal with land routes, which it could maybe harass with its air force, but it would have to deal with the PAF picking off bombers every chance it got. Taking all that into account, a land blockade would cause India to suffer a lot of casualties. On the sea front, it simply wouldn't be worth bringing the navy into the fight, as India's naval asset (while huge compared to Pakistan) aren't numerous enough to mount such an operation in a short enough time to be effective. The only thing, when it comes to the naval front, India would be interested in is taking over Pakistani ports to start an invasion from the sea, which it simply won't be able to do.
So, can India blockade Pakistan through its navy? Yes, but there would be no point.