Przed spływem kajakowym wkładamy telefon do worka wodoszczelnego.

Because przed requires the instrumental case here.

• basic form: spływ kajakowy
• after przed: przed spływem kajakowym

Both words change because they are both in the instrumental singular:

• spływ → spływem
• kajakowy → kajakowym

So przed spływem kajakowym means before the kayaking trip / before the canoe trip.

Spływ kajakowy is a very common Polish expression for a kayaking/canoeing trip, especially a trip down a river.

A spływ is literally a float / drift / descent downstream, so this phrase often suggests a river trip rather than just kayaking in general.

They often do, but some combinations are commonly used with the adjective after the noun, especially in fixed expressions or more technical/classifying phrases.

So:

• spływ kajakowy = the usual, natural phrase
• kajakowy spływ = possible, but less standard in this context

Here kajakowy is classifying the type of trip, so spływ kajakowy sounds very natural.

In Polish, the present tense is often used for:

• habitual actions: we put
• instructions: we put / we place
• describing a usual step in a process

So wkładamy telefon do worka wodoszczelnego can sound like:

• We put the phone into a waterproof bag
• Before the trip, we put the phone into a waterproof bag
• Before the trip, you put the phone into a waterproof bag in an instructional sense

This is very normal in Polish.

Wkładamy comes from the verb wkładać, which is an imperfective verb.

It literally means to put in, to place into, to insert.

So:

• wkładać = to be putting in / to put in regularly
• wkładamy = we put in / we are putting in

The perfective partner is włożyć. For example:

• Włożymy telefon do worka = we will put the phone into the bag / we’ll put it in once

Because telefon is the direct object, and for a masculine inanimate noun in the singular, the accusative usually looks the same as the nominative.

So:

• nominative: telefon
• accusative: telefon

That is why you see wkładamy telefon.

Because do means into / to and it requires the genitive case.

The noun is:

• basic form: worek = bag
• after do: do worka

So:

• wkładamy telefon do worka = we put the phone into the bag

This is a very common pattern:

• do domu = into/to the house
• do pudełka = into the box
• do kieszeni = into the pocket

Because wodoszczelnego has to agree with worka, and worek is in the genitive singular after do.

So:

• base form: wodoszczelny worek = waterproof/watertight bag
• after do: do worka wodoszczelnego

Both words are in the genitive singular masculine:

• worek → worka
• wodoszczelny → wodoszczelnego

They are similar, but not exactly the same.

• wodoszczelny = watertight, sealed so water does not get in
• wodoodporny = water-resistant / waterproof in a broader sense

For a bag protecting a phone during a kayaking trip, wodoszczelny is very appropriate because the idea is that water should not get inside.

Because do is used for movement into something, while w is usually used for being inside something.

Here the action is putting the phone into the bag:

• wkładamy telefon do worka = we put the phone into the bag

If you wanted to describe location instead, you would use w:

• Telefon jest w worku = the phone is in the bag

So the difference is:

• do = direction, movement inward
• w = location inside

Yes, that is grammatical and understandable.

• przed spływem kajakowym = very natural
• przed kajakowym spływem = also correct

The version in your sentence sounds especially natural because spływ kajakowy is a common set phrase.

No, Polish word order is fairly flexible, but different orders change the emphasis.

Your sentence:

• Przed spływem kajakowym wkładamy telefon do worka wodoszczelnego.

This sounds natural and neutral, with the time phrase first.

Other possible orders:

• Telefon wkładamy do worka wodoszczelnego przed spływem kajakowym.
• Do worka wodoszczelnego wkładamy telefon przed spływem kajakowym.

These are also grammatical, but they put emphasis on different parts of the sentence.

In context, it can work both ways.

Grammatically, wkładamy literally means we put. But in practical use, Polish often uses we for general instructions, similar to English expressions like:

• First we open the box
• Then we put the phone inside

So here it may mean:

• we put the phone in
• or more generally, you put the phone in / one puts the phone in as an instruction

That is very common in everyday Polish.