Worek wodoszczelny leży pod dziobem kajaka.

It is in the nominative singular, because it is the subject of leży.

• worek = masculine singular nominative
• wodoszczelny = masculine singular nominative

In Polish, adjectives agree with the noun they describe in gender, number, and case.

They usually do. The more neutral everyday order is wodoszczelny worek.

But worek wodoszczelny is also possible. Putting the adjective after the noun can sound a bit more classificatory or label-like, as if naming the type of item: a waterproof bag / dry bag.

So both are correct, but wodoszczelny worek is the more basic pattern to learn first.

Leży comes from leżeć and means lies / is lying.

Polish often uses posture verbs where English might simply use is:

• leżeć = lie
• stać = stand
• wisieć = hang

So Worek wodoszczelny leży pod dziobem kajaka sounds natural because the bag is understood as resting somewhere. Jest pod dziobem kajaka is grammatical, but less specific and less vivid.

Because pod can take two different cases:

• instrumental for location: under
• accusative for movement toward a place: to under / underneath

Here the bag is already there, so this is location, not movement. That is why Polish uses the instrumental:

• pod dziobem = under the bow
• Wsuwam worek pod dziób kajaka = I slide the bag under the kayak's bow

Two things are happening:

1. The case ending changes.
   In the instrumental singular, many masculine nouns take -em, so dziób becomes dziobem.

2. The vowel changes from ó to o.
   This is a common alternation in Polish:

   • dziób → dzioba, dziobem
   • compare stół → stołu

So dziobem is simply the instrumental singular form of dziób.

Because kajaka depends on dziób, not directly on pod.

The phrase dziób kajaka means the bow of the kayak. This is a very common Polish pattern:

• noun + genitive noun
• X of Y

So:

• dziób kajaka = the kayak's bow / the bow of the kayak

The preposition pod governs dziobem, while kajaka is genitive because it shows a possession or part-whole relationship.

Yes. Dziób literally means beak, but it is also used for the bow / prow of a boat or kayak.

So:

• dziób ptaka = a bird's beak
• dziób kajaka = the bow of a kayak

This kind of extended meaning is very common in Polish.

You do not know from grammar alone, because Polish has no articles like a and the. Context decides.

So the sentence could be translated as either:

• A waterproof bag is lying under the bow of the kayak
• The waterproof bag is lying under the bow of the kayak

If Polish wants to be more explicit, it can use words like ten for this / the specific one, but very often that is unnecessary.

No. Polish word order is fairly flexible because endings show grammatical relationships.

This sentence has a neutral, straightforward order:

• Worek wodoszczelny leży pod dziobem kajaka.

But other orders are also possible:

• Pod dziobem kajaka leży worek wodoszczelny.
  This puts more focus on the location.
• Wodoszczelny worek leży pod dziobem kajaka.
  This uses the more usual adjective-before-noun order.

The basic meaning stays the same, but the emphasis changes.

Not exactly.

• wodoszczelny = watertight, sealed against water
• wodoodporny = water-resistant or sometimes loosely waterproof

For a bag that is meant to keep water out, wodoszczelny is the stronger and more precise word. That is why it fits well here. A phone or jacket might be described as wodoodporny, but a dry bag is very naturally wodoszczelny.