Neural network to detect one class of object only
I'm not sure if I understand your question well.
Single class training data maybe not exist at all. If you want to detect only the sea cucumber, it is the two-classes classification problem right? It is the sea cucumber or not. Yes or no are two classes.
Yes right cool people implement the NN on raspberry pi. But to some extent it's just possible but not efficient. A good GPU will speed up much for training.
A PC is enable to train some small NN.
I'm totally new with neural networks (NN) in python, and I do not know if NN can run in raspberry pi 3? for I think the problem is that NN requires good CPU/GPU performance for training , data transferring and calculation. 
So is it possible to train a NN with single class training data? inorder to save CPU/GPU?. 
For example I want the system to detect only the sea cucumber in an image.
A good answer/explanation or link to any example will be very appreciated. 
THANKYOU PO