I'm kind of stuck with making a poster at the moment and I really need help TnT

So I was told to make a poster about aquatic ecosystem for my ecology class and I have no idea how to use photoshop (I dont even have it in my laptop) so I'm really in need of help from someone who could make me a background for it

I can't request that from a shop since it's not for a story so I'm stuck TnT

I really hope someone could help me with this

I would be thankful for you forever <3