Thanks for the flattering words, M4w!
It is difficult to tell size on cannons without anything to give scale, but there are a few pointers.
First very unlikely to be a swivel gun. It does appear to be too large for that and swivel guns don't need dolphins.
You can get dolphins on late 18th and early 19th century but they are a different profile- usually more square and without decoration- handles rather than dolphins. This example has more rounded profile and if you look very closely, they do appear to be in the shape of fish. As a rule of thumb, the more decorated your cannon, the earlier it is likely to be. (except for the earliest bronze cannons which can be very plain).
Not a howitzer. They are shorter guns with big calibre to length of gun. This one appears to be in the normal proportion for a regular cannon. And howitzers are more land guns than sea guns, although of course they get transported, so can end up in the sea.
I was not sure of the barrel is decorated or damaged. A straight rather than flared muzzle suggests earlier than later.
Finally your picture is a good match. Normal naval ordnance of a western European power with enough money to buy bronze guns in the 16-17th centuries. (But probably not Portugal or Venice).
Smithbrown