Terrorists are very creative and versatile and are always researching and studying new tactics to strike where least expected. I envisage a scenario of a team of sea divers with a high explosive suicide vest and or high calibre weapons storming into a beach or naval facility and wreaking havoc. It gets worse if it is a seaside metropolis. A form of this suprise attack was carried by jihadists from Pakistan when they attacked Mumbai in India though this time they were less discrete and arrived in dinghies.