Have a look at the used section. 500$-800$ can be very little for a good housing and compact. In fact the TG5 with Olympus housing is probably one of the few that sits in that price region. It is a good camera, but if you have a look at 1" sensor compacts (Sony RX100 (any model really), Panasonic LX10, Canon GX7mkII) you will find that while they may not be quite as good on the macro side as the TG5 they are significantly better for wide angle. If macro is your main interest though, you cannot go wrong with the TG5.
When shooting with compacts I find that even for Wideangle the smaller and cheaper strobes are good enough. I'm sure more light is better, but I find that I can get away with my YS-02 (same as YS-01s without TTL). I would recommend shooting manual strobes from the start. TTL is a crutch, and one that is not very reliable.
Your Accessories budget is probably already filled with your strobe arms. I doubt you can squeeze in an extra lens. I think you will be fine without a wetlens starting out though, so I wouldn't stress about that. My first attempts at UW-Photography were all with a bare Panasonic LX7 and I'm still happy with some of the images.