Here's the aircraft carrier mode. It's actually a twin-hull catamaran-style ship, and about the only thing distinguishing it from a ferry is the control tower, which is the accessory with some bits of plastic pulled up and folded out.
The view in the centre showcases and brackets the control tower. I haven't modelled the connection for the accessory - it might be a peg, or even a magnet. Note that this is the mode with the widest main hull so far - it's actually three times as wide as the train mode.
The lower right shows a more side-on shot, to see how the tower looks from the other side of the ship. It also makes it clearer that the landing strip area on top actually juts out over the back of the ship.
The underside shot on the left lets you see the twin hull construction, and also where the weapons pod has been secreted.
Play value: The sensor array on top of the tower rotates and the four spikes rotate up and down (all transformation joints). For attack mode, slide the landing strip forward and fold it up, swing the weapons pod up from the back of the ship to the top deck, and optionally tinker with the sensor array. The jet thrusters are actually on the back of the twin hulls, too, although I'm not sure yet whether they should always be exposed or have panels covering them when they're not needed. I've included a shot of the attack mode (from behind) at the top right. It's kind of the aquatic version of the H-tank. I haven't got around to remodelling the thruster vents yet after re-doing all the joints and components, so they're not shown.
Transformation: Basically, take the truck mode and flip everything that's underneath it out to form the sides of the aircraft carrier, then attach the accessory and pull out the sensors. (If the truck wheels were still deployed, they'd be marching in two lines down the deck on either side of the orange landing strip.) Rotate the windshield halves to form pontoons and that's basically it. There's a fair bit more to it, because the main body is further forward than in the truck mode, so another sixteen or so joints have to come into play and there's a lot of fiddling, but the underlying concept is 'take truck, pull undersides out to sides'.
I'm not 100% sure about this mode, given that most aircraft carriers aren't twin-hulls. Maybe it's just a large catamaran with a helipad? Maybe the attack mode would work better as a harbour patrol speedboat, with the sensor array becoming a front-mounted weapon of some kind? The landing strip/pad could actually rotate around a joint at its centre to assume the same angle as modern aircraft carrier runways, but it'd partially expose the hole at the rear of the vessel that you can see in attack mode.
Anyway. Thoughts?