Both movies are very when it comes to characters, and themes. Both movies have a catastrophic event. That causes terror and chaos.

Both heartbreaking romance stories. Both have an amazing storyline. Both deal with a terminal illness.

Both movies focus on a love story, both have a male character that has a dark past, both have a female lead that has an handicap. Both couples have to overcome tragedy

Both are excellent crime/investigation jdramas and both have Kimura Fumino as female lead.
Siren focuses more on the romance, the case is good and twisted. Ishi no Mayu has no romance but darker even more twisted cases.
both recommended

In both kdramas the theme is used: Money is power.
Both deal with corruption and murder. Somehow Signal ended for me where the story of The K2 might start.
2 of the best kdramas that got released in 2016
definitely a must watch!