Hi Nelson,
I don't know what the fishing is like in the creeks but I used to live just N of Mombasa. I'm not sure what the rules are today on beach fishing but the area between beach and reef is quite shallow and the snorkellers won't take too kindly to beach casters.
If you can get on one of the boats that heads down towards the Pemba Channel you'll get deep sea fishing you'll never forget.
I don't think you'll have a lot of luck on Kilifi creek but there used to be good fishing further up the coast at Lamu
Brian