Can I fish off diani beach and what would kind of tackle will I need
The problem with Diani beach is that the coral/rock pools comes almost right up to the high tide line which makes fishing far more difficult. At Galu beach it is very different as fishing is much easier. I'm afraid I'm not the expert but my husband does some fishing when we stay. He just buys a cheap rod off Amazon etc. so very basic and then leaves it for a beach boy when we go. Don't expect to be catching loads of big fish though.
I'm assuming you mean deep water sport fishing. This can be arranged at any of the established Diani hotels. I can't estimate the charter fee or speak to the quality of the experience though.