UNRELATED to Lightning. The flash miscaptioned as lightning bolt in the shared video is an igniting Det Cord (Detonation Cord) before reaching the explosives, which is part of controlled underwater explosion. The video was previously uploaded in 2012 by Rannikon Merityö, a Finnish water engineering company.
More in the EXPLANATION and REFERENCES section.
CATEGORY: False context.
SOURCE: TikTok, https://archive.ph/6dG0a (backup archive).
NARRATIVE: “Lightning striking water”.
Based on 7 Types of Mis- and Disinformation by First Draft News, classified as “False context: When genuine context is shared with false contextual information.” 
The SOURCE shared video of an igniting Det Cord (Detonation Cord) with false contextual information, which has caused a FALSE conclusion.
Identical video, Rannikon Merityö in Dec 21 2012: “In this video, we deepen the waterway.” 
Other Related References
PdL Support Systems: “Detonation Cord consists of a high-explosive (either PETN or RDX) core wrapped in a reinforced, waterproof, olive-drab plastic coating that transmits a detonating wave. It is used as a detonating agent, a priming agent or alone as an explosive charge. It can be initiated by an electric blasting cap or non-electric blasting cap. When the explosive core is detonated by a blasting cap, the wave travels along the cord to other blasting caps or explosive charges attached to it. It has a detonating velocity of not less than 5,900 m/s which makes it suitable for synchronizing multiple charges, even when placed at different distances from initiation.” 
FullMag: “500 ft of Det Cord”. 
Google – Video search result. 
turnbackhoax.id: “In fact, it is not a natural phenomenon but rather man-made. The video is part of a service provided by a Finland-based dredging, quarrying and coastal construction company.” 
 firstdraftnews.org: “Understanding Information disorder”, https://bit.ly/34P3vBc / https://archive.md/02Dro (backup archive).
 youtube.com: “Porapaalutusta, osa 3: Räjäytys”, https://bit.ly/3Irtbpd & https://bit.ly/3rIrixI (Watch Frame by Frame) / https://archive.ph/sxnbt (backup archive).
 pica.army.mil: “Detonation Cord”, https://bit.ly/3IIEqtR (archive) / https://archive.md/DqXDu (backup archive).
 youtube.com: “500 ft of Det Cord”, https://bit.ly/3FNo1lL / https://archive.md/7fEU9 (backup archive via Watch Frame by Frame).
 google.com, https://bit.ly/3AoKvbV / https://bit.ly/3nN29Ru (backup archive).
 turnbackhoax.id: “[FALSE] Video of Natural Lightning Striking River” (Google Translate), https://bit.ly/3FSVA5Q / https://archive.md/ZVgnc (backup archive).