Re: no matter what it will average out over time, as you know
As noted before:
Given the 1 fish attempt every 10 seconds.... some modifications...
360 fishing attempts/hour
The number of non-usable fishing results, IMO, is higher in water that can spawn serpents, especially when you factor in time lost dealing with serpents (including skinning/looting them), even if your pet/summons does the fighting. Dungeon Fishing doesn't have the serpents, so would probably be slightly better for successful casts per hour. We'll assume the 1 in 6 being bad as a general number.
The biggest hassle in all locations is getting a special fish, that you got to dump on the deck or in your hold, to keep you from going overweight. If you're not using a "target relative" macro (or one for a specific target, if you go the casting into 4 specific resource squares every trip route) for the fishing part, cutting the big fish/rare fish up can screw with your targeting for fishing.
Even so, with no special fish, you're still going to have to unload (into a hold, beetle or other pack animal) every 4-7 minutes, ESPECIALLY to get rid of the accumulating footwear - even the UOAssist organizer agent takes a second or more per item to move into a bag on you, then you manually have to drop the bag), which takes a minute or so if you're not using illegal software, since the new fish don't auto-stack on deck (unless Tugg's done something in the last few weeks I've not heard of to rectify the situation with UOAssist).
Assuming that you lose the equivalent of 5 casts with every stoppage to unload, that would average the loss of 60 casts per hour.
As we noted above, there would be another 60 casts an hour lost to the actual "catching" of footwear, serpents, big fish and messes of small fish
Of all these, only the fish steaks off Big Fish & normal sea serpents, and the horned leather off serpents, auto-stacks when cut with a proper tool. While the Harvester's Blade, while it loots the scales, the scales DON'T STACK with existing piles, and you get 10% or more extra leather, scales PLUS the unwanted meat (for deeps - the fish steaks might be considered unwanted by some), and still have to manually loot the gold and/or special loot inside the serpent corpse.
The actual number of fish caught, therefore, is probably closer to 240 to 270 an hour, or (if evenly distributed, which almost never happens)
20 of each shallow water or dungeon fish an hour;
13 to 15 of each deep water fish each hour.
HOWEVER, even these numbers are misleading, for Dungeon fishing has load issues.
If you're dungeon fishing, you don't have a ship hold to drop into. This means that you are either dropping into containers as a sorting method (then using a Bag of Sending to send each type to the bank, in batches of 40-50 at most), or have a trained giant beetle with you for storage. The latter only holds 160 fish, unless you play with the fish types that resemble the old "a fish" (3 of the dungeon types, 4 each of shallow and deep) to deliberately get those fish stacking at 1 stone (not gonna say how to do that - you can figure it out yourself).
This means that, unless you're one of the following special cases, you'll have to leave the dungeon at least once an hour to unload, assuming you are bringing out ALL of your quest-related fish intact.
1. Fisher, alone: will get full in less than 8 minutes. figure 6 unload trips an hour.
2. Necro Fisher: Gets an additional 7 minutes from using a horde minion. so, figure 4 unload trips an hour IF you can gate, or have someone that gates to you (not possible in Ilshenar or Fel dungeons, as well as a few elsewhere). Without the ability to gate out, you lose a quarter of each hour going back and forth (most no-gate dungeons take about 2-3 minutes each way to get to where one can gate out, be it a cast gate or taking a permanent ilshenar gate to a different facet)
3. Typical Fisher with beetle: Beetle only holds 160 fish if you don't trigger the bug that causes certain fish types to default to 1 stone weight like their pre-HS visual counterparts. I'm not gonna say how it's done, but you can probably figure it out if you don't know already. even so, that only allows 1/4 of your fish to get down to 1/10 normal weight, so you're STILL going to have to go unload at about the 50 minute to 1 hour mark.
Special cases:
4. #2+#3 above: Gets you an extra 40 fish over and above your pack and the beetle, so you MIGHT be about to go 10-15 minutes past the 1-hour mark.
5. Fisher/Vet: You take 5 bonded packies in, not caring if they die. You sort on the ground/bags as you fish. When you leave, you clear out the spawn (if any) that's wandered in, rez your pack animals, load each up with 160 fish (more or less - you might catch other fish to keep, or get the fish to stack at the lower weight), without the containers you sorted into, then recall out. You can get about 3-4 hours fishing in this way.
6. multi-packies: A non-vet can try something similar to #5 by leaving the bonded packies at home, logging out in the dungeon then logging back in, to get them to the loading spot. Yes, you can have 5 bonded packies with only 4 stable slots, by leaving one in your house when you log out. Typically only recommended for resource gatherers, and even then you occasionally lose the one logged out from forgetting to completely unload it, or logging out too close to backups prior to scheduled shard drops or unplanned crashes.