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You have proposed an interesting project. Though you'll need to figure out the specific complexities on your own, I think I may be able to help point you in the right direction..well, at least A direction. There are many ways to achieve what you are asking.
The first thing I would consider is the abutments at each end. Typically, this would mean building a retaining wall to stabilize the banks. To do this you would need to drive posts in at the bank then plank the soil side of the posts. Next fill in the area nice and level using good soil then compact thoroughly. Excavate an are about 4'x4'x1' behind the retaining wall and frame a square box using 2x12's then fill the box using 21AA crushed limestone or similar, compacted every 4".
At this point, I would determine my fully required distance at the far ends of the new abutments; If it was originally a 50'-0" span it would now be 58'-0" distance.
As suggested above, I would consult a span table keeping in mind that you will want to use 0.60 pressure treated southern yellow pine lumber throughout. Given that, I would consider using 2x10 joists @16" on center. I would shoot for a relatively low modulus of elasticity so the bridge doesn't bounce nor sag over time. I would break up the spans to make optimal use of 16ft and 14ft lumber, which would give me spans of approximately 11ft and 12ft plus a short central span of about 4ft.
The depth of your pond will ultimately determine your post dimensions and material. I figure for a typical 6ft deep pond, 4x4 posts will do but you may also consider using steel beam or pipe. I would figure on no less than four pairs of posts with headers on both sides and cross bracing.
Going well above and beyond code, I would double the outer joists and stagger them to overlap by half to create straight, solid 3" wide beams. I would also add 3' block cleats to solidly join each inner joist. Last, I would block in between the joists every 8' before attaching deck planks.
I think it should be clearly stated that there will undoubtedly be specific site circumstances that must be considered; depth of water, density of soil below water, stability of soil at banks, actual vs. approximate distance between banks, local codes, insurance liability, quality of materials available, added mechanical connections required, etc,etc. This is not something you can just do on a whim and complete in a weekend but at least you have a starting point for when you consult your local engineer, builder and county officials.
I am including a sketch of what I mentioned above. Please do not consider it a plan to build from. Take my advice at your own risk.