Oh boy, dolphins. This mission has some naval crap going on like others I dislike. This is somewhat difficult as you need to protect a large coastal area. And you'll have to have dolphins to do it. Build a normal navy but also build some annoying dolphins to protect them from those equally annoying squids.If you want to go cheap you could build a huge army of dolphins, as they can hit any naval unit hrm. Anyway, on to more interesting things. Try to have some good pillbox coverage to protect against engineers, there are some huge gaps in base defense right now and you need to fill them. Strive to take control of the seas. After a while, the map will expand and you will be able to launch attacks against the shipyards that are sending these vessels to harass you. You can achieve total control and they will not be able to attack your base again. But, you have the 20 minute countdown until they use the nuke.

Construct a couple of aircraft carriers to attack their static base defenses and prepare to use some transports to bring in some prism tanks. You can also use the chronosphere to do this if you have it available in time to use it. Bring the tanks into an undefended area on the coast or behind the base (chronosphere). If you take out both of the nuclear reactors with your carriers they won't have enough energy to power the nuclear bomb - or anything else for that matter. After that, it's a walk in the park for your prism tank force.