Assembly and coupling elements for single-seater or multiple-seater pedal-powered watercrafts having paddlewheels, constituted by at least a pair of identical crossbars, which are arranged in the same horizontal plane perpendicularly in relation to the direction of advance of the boat and having an adequate length for enabling that they may be fixed to a number of identical main longitudinal floats, all of them longitudinally arranged in relation to the direction of advance of the boat in the same horizontal plane and being all of them symmetrical in relation to their respective vertical longitudinal symmetry planes, being this number of main floats equal to the number of desired passengers plus one, and there is enough width between each two adjacent longitudinal main floats for placing here all the necessary pieces, accessories and elements for accommodation of passengers and for propulsion and control of the boat, with this floating structure obtaining rigidity with two equal U-shaped longitudinal bars or plates for each seat, and then for each pair of longitudinal main floats, that are placed between each two adjacent longitudinale main floats and symmetrically in relation to the vertical central symmetry plane of the same pair of main floats, with such three structural elements characterised in that they enable assembly of two-seater and multiple-seater pedal boats by using as many number of main floats as number of desired passengers plus one in the watercraft to assemble, and later by repeating and joining sidewards all the necessary pieces, accessories and elements for accommodation of a single passenger and for propulsion of the boat as many times as the number of desired passengers, in a way that in two-seater and multiple-seater watercrafts each passenger shares a main longitudinal float with the adjoining passenger, so that the longitudinal main floats are fixed with screws or pins at equal distances between each two consecutive ones of them, further characterised in that from these three types of pieces, the crossbars have different standardized selectable lengths for each number of passengers or seats in the watercraft to assemble, so by such feature they make possible the assembly of watercrafts for different number of passengers by enlargement sidewards, always assembling the main floats with a constant transverse separation distance between each two adjacent ones of them, and also between their respective vertical longitudinal symmetry planes, further characterized in that the U-shaped longitudinal bars or plates are joined and intercrossed in right angles with both main floats and crossbars for increasing strengthening with a triple union of these three elements arranged in three respective horizontal parallel planes, being arranged each pair of such U-shaped longitudinal bars or plates over and between each pair of adjacent main longitudinal floats, each one at one of the sides and symmetrically in relation to the vertical longitudinal symmetry planes of both adjacent main floats, and over the crossbars at the same time, resting in contact with and being joined to these two pieces, further characterized in that each U-shaped longitudinal bar or plate has its own middle straight longer longitudinal segment arranged in longitudinal direction in contact and joined perpendicularly to all crossbars in a selected chosen preset transverse position, and its twin transverse shorter segments connecting in transverse direction the previous longitudinal segment to the main floats where they are joined to them, further characterised in that such three structural pieces are joined by three different groups of vertical screws or pins (7,13 and 16), each one of which joins mutually each two of such types of pieces between them, and are placed passing through coincident consecutive vertical holes (5, 6, 11, 12, 14 and 15) practiced in the common surfaces in contact.