An Octopus? A belt with old pantyhose stuffed with newspaper to fill. Then you can papermache the outside and paint an appropriate colour and wear pants/top in the same colour...

Green seaweed. Wear green and pin green streamers to the clothes.

Starfish - can be any colour. Two large pieces of cardboard cut in the shape of a star and then some string at the top to make a frame type thing to just hang over head to sit on shoulders...