Getting better at the renders! I would however advise you to get rid of that thing in front of the sail (the building on the "hull"), it makes your sub look weird, no offense. Also, make your sail ovular, like on mine. Overall, try looking at my sub and try to find details that make it stand out, and try improving yours with those details. Following your download, your hull seems too much tiny, in my opinion.
Try modelling with blocks. This is what helped me model my boats better. The vid is fairly nice and should provide enough info on how to improve your boats. Of course, everyone's style is different, so learn to develope and improve yours. Good luck and hope this helps!