Avoid OpenCL. If you're going to the trouble to use an FPGA, it means you really want performance. In that case, there's no avoiding the rewrite in VHDL/Verilog and specifically targeting the resources that your FPGA hardware offers and rewriting your algorithms accordingly. The platforms are not the same.

e.g. 32-bit floating point is what GPUs are made for, while FPGAs really only handle integers unless you've purchased an expensive one with specialized arithmetic units on chip (e.g. DSP48 slice for Xilinx).