Well, your first mistake was going with nikon! Haha, jk, but I do hate nikons with much passion.
The lens you have is usable for macro photography, it's a simple fix, buy extension tubes. These tubes lower the min focusing distance of your lens, and allow you to move in closer. I believe that the canon 18-55mm has a min focus distance of like 10" inches I want to say, with the addition of extension tubes, I was able to move in super close and get this shot, which is honestly, my favorite macro I've ever taken. These tubes run about $100 bucks, I got mine from a friend who never used em...
https://fbcdn-sphotos-a.akamaihd.ne...318948181_223552384355508_803843_580498_n.jpg
So you can buy a set of these and buy them for your lens till you can afford the true macro that you want. the only down fall to using these is you lose some light with the extension, however this issue can be fixed using exposure compensation or the use of a editing program like photoshop or lightroom and bring the exposure of the photo up in post production.
Shooting from above is hard work, but the others covered that really well, there is a sponsor on this site who makes a product that you can attach to your lens and it will allow you to take pictures with the lens partly in the water, check out avast aquatics on here, they are the ones who make this product.
if I were you, at this point, purchase some extension tubes for use with your lens, and save up for a GOOD macro, don't waste your money on the small 50mm macro, that's not what you truly want and you know it, buy what you want when you can afford it, the extension tubes are a cost effective way to get macro shots without have to sound near the cost of the 50mm macro, so save up and buy a 105mm or a 180mm macro.
And don't use flash on your pictures, it will wash the color out on the corals so badly it's not funny, some people use it by lowing the flash compensation in the camera, but for me it's nothing but a issue...just learn how to manipulate your cameras settings and get the color as close to realistic as possible, then edit the colors in photoshop or lightroom.
Good luck!