Dear Mr. Coward (I bet that name has never caused you issues in your life)
Yes it's absolutely fine to do it.
Only, in future you need to be dressed in black and choose a moonless night, probably around 3.00 am.
There are a couple or three addresses I mean hedges backing onto the towpath at Milton Keynes which deserve it. Another really suitable place is a housing estate near Glasgow. It's nowhere near a canal, and takes a bit of a bus ride but this particular bloke sold me a car ten years ago knowing full well it was a bad 'un.
Some people think human excrement damages the health of the fairies at the bottom of the garden, when actually it's the Toilet Chemicals which can be a tad worrisome. If you follow This Link you'll find The Safety Data Sheet issued by Elsan for theirs.
If you haven't the inclination to be bothered to click on it, similar to that needed to dispose of a toilet's contents properly, then I'll detail a few of the lesser issues.
Main hazards Toxic by inhalation, in contact with skin and if swallowed. Causes burns. Limited evidence of a carcinogenic effect. May cause sensitisation by skin contact.
Elsan Blue toilet fluid
2.2. Label elements
Acute Tox. 4: H312 - Harmful in contact with skin. Skin Irrit. 2: H315 - Causes skin irritation. Skin Sens. 1: H317 - May cause an allergic skin reaction. Eye Irrit. 2: H319 - Causes serious eye irritation. STOT SE 3: H335 - May cause respiratory irritation. Carc. 2: H351 - Suspected of causing cancer .
Precautionary Statement: Prevention
P102 - Keep out of reach of children. P262 - Do not get in eyes, on skin, or on clothing. P280 - Wear protective gloves/protective clothing/eye protection/face protection.
Precautionary Statement: Response
P301+P312 - IF SWALLOWED: Call a POISON CENTER/doctor/ physician /if you feel unwell. P302+P352 - IF ON SKIN: Wash with plenty of water/ soap and water. P304+P340 - IF INHALED: Remove person to fresh air and keep comfortable for breathing. P305+P351+P338 - IF IN EYES: Rinse cautiously with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing.
So as you can see an issue or two for others to be bothered about ....... p'raps. However as it's not your land you're tipping the above on, plus your excrement onto why give a Flying Fox eh?
Being aware of your effective and I imagine well practised technique perhaps you could offer a service to others. If when passing your moored boat it would be convenient for us to empty our bogs down your chimney, for you to dispose of later?