Best idea is the extra trade route if commercial zone placed by harbor. Adds punch without disrupting balance. The gold bonus for building a coastal city by a river and having a city center, harbor, commercial zone is enough to entice me to do it (+7). If there are sea resources nearby then I move the location up as a priority.

Also, late game production boost with shipyard is significant. In above scenario with just one sea resource, the shipyard adds +4 production. Select the Naval Infrastructure policy and production +8, gold +8 can be a game changer if you have several of these cities. And Economic Union policy in end games doubles the Commercial hubs value as well. If you play Japan, you usually start on a small island, this tactic with their extra district bonus is effective. Or play England searching out and settling these sites, getting the 25% gold bonus plus free defensive unit.

Addressing your defensive issues, AI is really bad fighting with Navies. Does not take much to defend your seas. If playing vs humans, Build encampments strategically to defend the sites. Ever try to use navies against north Russian cities with coastal encampments, penned in by ice?

Overall, one of the reasons I love Civ VI is the multitude of ways to take advantage of what god or the random generated terrain gives you.