To answer your rudder question, yes of course you can use your plastic kayak on the sea without a rudder. I have a Fluid Chumani plastic ski which has no rudder and I'm a lady fisheman who fishes in the sea, my friend "Loco" on Sealine has an Ocean Kayak plastic sit on top kayak which she fishes from on the sea and a few guys who lauch off the Bluff have Perception Swings and Feel Free's which are also plastic sit on top kayaks, which all also have no rudders and they work just fine, I promise, you have nothing to worry about in the slightest.
The Chumani seats one paddler, with hassle free molded-in footholds that can accommodate a large range of paddler sizes. Four drain holes get rid of water that splashes in the boat, preventing the boat to turn into a bathtub. The boat have four handles, which, combined with the short length, makes it easy to handle out of the water.
A sealed hatch behind the paddler keeps items safe and dry. The recess on the stern is designed to hold most regularly used cooler boxes as well as a wide range of dry bags and fishing bags. The compartment on the bow can also hold a variety of dry bags, and can also serve as seat for a kid facing the paddler.
The outfitting is finished off with a drain plug for the unlikely event of water leaking into the boat, and a range of webbing loops where optional thigh straps or backrest can be attached.