Prudence - Hazards on the water

Logs
On some waterways, at some times of the year, there can be lots of substantial chunks of trees, saturated and floating well down in the water. In quieter* rivers like the Marne they require looking-for and avoiding. In fast-flowing rivers like the Seine they can also present a significant hazard when one is moored-up. We were hit by a big log whilst pontooned at Rouen and avoided being hit by one twice that size by lassoing it and hitching it to the pontoon support column. Travelling slowly in 3.5m+ depth approaching Meulan we clonked something hidden below - and it was not a shopping trolley!.
Along the Marne there are lots of fallen trees along the riverside, some of which must surely project out 5-10m into the channel.
[* During winter-spring, all navigable rivers are liable to be fast flowing, in full spate].

Depths
Even on the mighty Rhone, with mid-channel depths of 3.5m+ (sometimes much more) there are shallows. As always, they can be in unexpected places (which term also includes the expected places one forgets about). Low islands and visible sandbanks almost always also mean there are hidden shallows around.

The Bends
The deepest water seems (usually, but not always) to be found on the outside of any curve or bend in the river. On the Rhone this can mean a difference of 3m or more.

Bridges
Bridge supports often have hidden bases that - nastily - step outwards below the waterline.

Banks
Many bank-sides are shallow (the canal or river is not a trough). Those that are not shallow are rocky, or have rocks one cannot see just below the surface. Those that are neither one nor the other, are often both. Midi Canal bank-sides are delightful, mainly consisting of a tangled web of tree roots, and shallows. (They are actually easier than other canals).
The accepted method for bank-side mooring (possibly for lunch) consists of gently setting the bow in, jumping off and setting a stake or (rond) anchor and then securing the stern - possibly letting it stay out where the depths are more congenial to rudder and prop. A pasarelle (gangplank) is often useful - we made ours from a cheap single section aluminium ladder and 3 decking planks. At least 1/3rd the cost of a chandlery-bought one. And smarter.

Boue
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Big Boats - Peniches, Ships and Cruise Liners

Bumper Boats
On the Midi, some (by no means all) hire boats can constitute a hazard. To navigation or to incident (damage) -free locking. They've got big rubber bumpers all round the boat and (because of lack of experience, or care, or both) they bump into things - walls, each other, us maybe. They also travel too fast - on the canal, in excess of the 8kph speed limit, and entering and leaving the lock (which reduces their capacity to control the vessel still further). We have been told that the hire companies (a) tell their customers not too worry too much about hitting 'private' boats because "they're all insured" and (b) set travel schedules for their customers that mean they have to keep pushing on as fast as possible otherwise they won't reach their destination depot in time and (c) do not advise customers about speed limits, nor limit the speed that boats can travel at - other than telling hirers "it's best not to exceed 2,000rpm" (which of course they can and do).

Be Prepared
We planned our passage across the Baie de la Seine to Honfleur, took account of tides, navigated and sailed successfully for some 11 hours. We then got ourselves and the boat in a pickle through not properly assessing what we would find at 'the other end' - a tide in the river itself that turns significantly later than to seaward, but which then flows strongly, that we would be tired and it would be dark, and that we would be entering an unfamiliar and substantial 'proper' lock, which was not something we were used to. The result of this combination of oversight and innocence was that the current pushed us into a big lock crossways, we got our ropes wrong, and because we were tired and the situation was dark, intimidating and unfamiliar we failed to extricate ourselves before the bow guard-rail hit the lock wall rather hard. Of course we then had to try and control or contain our dismay and overcome, which was not easy.
In Rouen, trying to be helpful to what seemed to be a reasonably competent crew on an adjacent boat ended up with our own boat out of control and in a certain amount of peril through no fault of our own, completely unexpectedly.
We guess one has to prepare for every aspect of what can be expected, but at the same time somehow cope when out-of-the-blue happens - as it will. Depressingly, expect the worst in every situation but be pleasantly surprised 99% of the time!