SPOILER SPOILER SPOILER SPOILER !!!

It wasn't real clear. He and Alex were the last ones on the sinking shot-up trawler. I watched carefully on my second viewing. Alex taps him on the shoulder (like to let him know to leave) and Alex turns and heads out of the trawler (which is now almost full of water). You see the French guy struggle underwater and he drowns - you just see his hand which is reaching under the water for the ladder, and his hand stops moving. According to Wikipedia, his ankle got caught in chains. Some say you can hear the clanking of chains while he is struggling to get out.

thanks for your answer. i already had second viewing and yes correct he drowned in that fishing boat. dont know why but on second viewing this movie was even more intense and the editing of this movie omg. 3 different lenght timeliness come as one but in such a genious way.

I bought the screenplay and that says the force of the water entering the sinking boat kept him from reaching the ladder (and air) and getting out.

oh wow. how can i get the script? :)

The screenplay is available on Amazon. I recommend the Kindle ebook which is about $8.99 (I think I paid more when it was first released). The ebook is easy to search for any word. Amazon offers free Kindle Reader apps for Windows, Mac/IOS, and Android devices. For some countries like US, Amazon also offers a web-based Kindle cloud reader so I can access any of my purchased Kindle ebooks (including the Dunkirk screenplay) from other computers (like my computer at work).

Not every country has access to Kindle ebooks -here is the main list:
Amazon.com: US https://www.amazon.com/Dunkirk-Christopher-Nolan-ebook/dp/B073VNL1F2/
Amazon.co.uk: Customers in the United Kingdom
Amazon.fr: Customers in France and Monaco
Amazon.de: Customers in Germany, Austria, Luxembourg, Switzerland, and Liechtenstein
Amazon.it: Customers in Italy, San Marino, Vatican City, and Switzerland
Amazon.es: Customers in Spain, including Canary Islands, Balearic Islands, Ceuta, and Melilla
Amazon.ca: Customers in Canada
Amazon.com.br: Customers in Brazil

I saw Dunkirk a third time. SPOILER SPOILER SPOILER

When only Alex and Gibson are remaining in the sinking boat, Alex starts to leaves, turns around, splashes back over to Gibson, (I think he) taps him on the shoulders and then Alex yells "Gibson! Leave it!". <- not in the screenplay script. Then Alex leaves the boat. You see Gibson try to follow but soon he is underwater and cannot get to the ladder. He drowns.

what i didnt understand, there seem to be other opening in ceiling which was much closer to reach than going all the way under water to reach the lader or I saw it wrong?

Yes there was some light coming in.. bit might be some transparent plastic or fiberglass hatch so it would be impossible to open it while swimming