#1. Make a backup of your profile before reading and doing anything below. :)

I've not played KotR for a long time now but I was a maniac player in the past. I only remember that you could detach the trailer with load and do something different but I have no idea if you're fined if the trailer is not delivered in time.
In my opinion, you might be OK. Look on your timer - it says 00:00. That might be a good sign that you can take another cargo and make some money.
Look on your F2-F4 tabs (I don't remember which one). There's a cargo or truck information somewhere. If there's no information about currently taken job than you should successfully take another one and forget that trailer.
But because there was a job in progress, be prepared mentally that you can be fined in any moment. Therefore, see #1.

The other solution (after making backup of course) is to reverse carefully to the water and spam "attach trailer" button while reversing 1 cm/s. I do remember you didn't need to be perfectly set with the fifth wheel to connect the trailer, you just need to be close enough. In case you connect, you can bypass ASR with some key and try to force the trailer out of the water. I cannot recognise on the image if it is the front of the trailer that is underwater near the shore. If that's the back door, you lost that trailer unfortunately.
The water puddles in KotR are a lottery. There's some of them that right after touching make you irreparable and some of them actually allow you to drive quite safely.
If you're OK with cheats in such rescue situation, just remember about FILLUP and TURBINE. :)

Please post your feedback if your game can be saved this way or not. I'm curious to see the results.

Lay the hammer down, pal! And have a nice day.