We consider a time-harmonic water waves problem in a 2D waveguide. The geometry is symmetric with respect to an axis orthogonal to the direction of propagation of waves. Moreover, the waveguide contains two floating obstacles separated by a distance L. We study the behaviours of R (the reflection coefficient) and T (the transmission coefficient) as L tends to +∞. From this analysis, we exhibit situations of non reflectivity (R = 0, |T | = 1) or perfect invisibility (R = 0, T = 1).