Rule 18 sub part e, of the open water Collision Regulations (col regs) apply here. It sort of common sense. Logic and col regs dictates that the less manueverable craft should have the right-of-way.
A vessel towing anything will have ultimate right of way. Sail boats and unpowered craft come next on the food chain. Then there are power boats. Howerver a sea-plane is less maneuvarable than a power boat/do not have reverse, limited steering on water. Unless the sea-plane has an open commercial lane, here is Col Reg 18 with part (e):


Rule 18

Responsibilities Between Vessels

Except where rule 9, 10, and 13 otherwise require:

(a)A power driven vessel underway shall keep out of the way of:

(i)a vessel not under command;

(ii)a vessel restricted in her ability to maneuver;

(iii)a vessel engaged in fishing;

(iv)a sailing vessel;

(b) A sailing vessel under way shall keep out of the way of:

(i)a vessel not under command;

(ii)a vessel restricted in her ability to maneuver; (dont go near the big ships...)

(iii)a vessel engaged in fishing; (they get really pissed)

(c)A vessel engaged in fishing when underway shall, so far as possible, keep out of the way of:

(i)a vessel not under command;

(ii)a vessel restricted in her ability to maneuver.

(d)

(i)Any vessel other than a vessel not under command or a vessel restricted in her ability to maneuver shall, if the circumstances of the case admit, avoid impeding the safe passage of a vessel constrained by her draft, exhibiting the signals in Rule 28.

(ii) A vessel constrained by her draft shall navigate with particular caution having full regard to her special condition.

(e) A seaplane on the water shall, in general, keep well clear of all vessels and avoid impeding their navigation. In circumstances, however, where risk of collision exists, she shall comply with the Rules of this Part.