The way the Sailor John becomes a sea monster scene goes in The Return of Sailor John.
[on the hidden rock]
Sailor John: Hmm... maybe I should attack that island... [pauses] ...as a huge sea monster! [cackles evilly]
[Stinky Pete gasps, and then shuts his eyes. Sailor John gets out the book and tears out the last page of the 'Thomas Story' and begins rewriting it, pausing to think a bit]
Sailor John: [Writing] Then.... ummmm.... uhhhhhh.... aha! 'Sailor John experienced a powerful transformation, as he turned into a huge scary.... uhhhhh.... sea monster! [puts the pen down and waits]
[a small moment of silence]
[A bolt of lightning strikes Sailor John, and it soon fades. The blast is witnessed by Dash, Bash, and Ferdinand on Misty Island, when it ends, Sailor John is normal.]
Sailor John: Arrrgh! Nothing happened! This thing didn't-
[Suddenly, Sailor John is engulfed in a bright white light and he begins to transform]
Sailor John: AAAArrrgghhhhhhh!!!!
[he glances at his hand. his hand starts turning green and scaly, and becomes webbed. His skin starts yellowing. Sailor John screams in pain as he falls to his back. He kicks his boots off, and they land on the rock.]
[Camera pans to his feet, as they begin to yellow, and then, his toes turn into thrashing tentacles.]
Sailor John: What-what's happening to meeeee-aaaaaargh!!!!
[Sailor John groans and growls as he grows bigger in size, tearing his clothes apart. a dorsal fin grows on his back,]
[Stinky Pete gasps in horror]
[Cut to his face, as his nose and mouth stretch longer, and his teeth turn long & sharp, like a shark's. His eyes turn a dark black colour, and his hair turns seaweedy green.]
[He slips and slides off the rock and falls into the sea, landing with a deep splash]
Stinky Pete: [gasps] Sailor John! Are you alright?
[Sailor John emerges from the water, looking completely different; his hair is longer, a seaweed green colour, and really messy, his skin is covered in yellow scales, and his eyes are black, and his nose and mouth is shark like, as is his teeth. and he is now 4 times his original size.]
Sailor John Monster: [demonically deeper voice] Raaaargh yes! So powerful! Just look at my new feet! [raises his feet outta the water]
Stinky Pete: [gasps] What happened to you?
Sailor John Monster: I have become a huge sea monster! feel the look of my new tentacle toes!
[On Misty Island, Bash, Dash and Ferdinand are watching]
Sailor John Monster: oh yes, Stinky Pete! feel the tentacle toes of my brand new feet! [wriggles his new tentacle toes]
[One of them touches Pete's nose]
Bash: [whispering] Ew!
Dash: [whispering] What has happened to him?
Ferdinand: [whispering] I think he turned into a giant sea monster!
[The three engines race to Sodor to warn Thomas]
Thomas: What was that noise?
Percy: I don't know.
James: And what was that bright white light?
Thomas: Bash! Dash! Ferdinand! What's wrong?
Bash: We saw Sailor John.
Thomas: What happened?
Dash: He turned into a sea monster and he's on his way here now.