Syfy movie with a giant octopus or squid terrorizing small town and killing people?

okay, so many many years ago I saw this movie on syfy about a giant octopus/squid wrecking a small town because it was killing people. The movie started off with 2 girls scuba diving and one girl is killed by the octopus and the other girl who happens to be her sister is with her and survives but is now determined to kill the octopus and she ends up getting help from this sci-ency/marine biologist guy who's lab partner has a limp and uses a forearm crutch to help him. another scene is where to young guys rent a boat to go scuba diving and end up being killed by it. sorry the description isn't very good but that's all I remember and also i know it's not the stupid syfy movie like mega shark vs giant squid so yea any help would be great!!!! thanks