My aquariums are in an unheated basement, and of course they have covers that help keep in warmth. I oversize the submerssible heaters to account fo rthe larger temp differential.
IMO sealing off an area and heating it with a space heater will be very expensive (inefficient heating electrical cost and I would worry about the "safety" of a cheaply produced mass market space heater, which can cause a fire or just fail. I would do not want to risk burning down the house.
Below grade, constant temp of 55 deg F is common so it will only get so cold in the basement. If you have a walk-out, then there should be insulation on the timber frame wall side. If you just have a bulkhead exit, get a steel insulated door installed it's economical.
You want to enjoy access to and viewing of yoour pond, so I would not enclose it and entice humidity and mold problems.