I'd say at least three feet deep for goldfish, four would be better. The pond isn't really going to be big enough to keep koi or orfe properly.
Five by eight feet gives you about 250 imperial gallons per foot of depth (assuming a flat bottom). Figure 30-50 gallons for each adult goldfish, and you won't be overstocked.
A marginal shelf is a good place for plants, but it's also a good place for herons to stand and fish. If you don't want to net the pond or rig other heron deterrents, you should consider making the minimum depth three feet.
I'm with you, I didn't think it'd be big enough for Koi. I read somewhere on here that you need at least 1200 gallons for them. I'd dearly love a sterlet (hence my name) but after reading the caresheets I couldn't offer one a good enough home.
My fish are colourful enough & as long as I keep in mind gallonage per fish (thanks for that too!) I can may be have some other colours in there.
I think I'll try a sloping bottom, one end 3ft going to 4ft. It's not herons I need to worry about it's cats (my own!) so I'll have a look into what deterrents I can use, probably similar to that for herons?
If you dug it to a constant depth of 4ft+ (formal pond) it will be suitable to stock with a few Koi. It will be more of whats called a "growing on pond" for the Koi, as they will EVENTUALLY outgrow it but believe me it will be a very long time. However if you aren't too confident with fishkeeping this method of keeping Koi probably wouldn't be the best route, so i'd stick with Goldies & the like for now.
Deterrents, sorry but from personal experience & other stories theres no such thing. Only way to put a stop to herons/cats etc is to put a solid mesh over the pond. Believe me deterrents may "seem" to work but it's only a matter of time before something will take your fish.
I suppose the best deterrent would to not have any cats! I wouldn't mind solid mesh if it keeps my fish safe :)
I've now got liner (with a 40yr guarantee) & underlay which is 19 x 16ft. The way I've worked it out it should give me & my fishies a reasonable sized pond. When I've got this set up properly I can then move onto the minefield of pumps, filters & UVs.