Hey Mawashi,
Theres a couple of boat hire joints up here,one i would recomend is gecko baot hire,there number is ( 08 ) 8981 2733, you might want to ring and organise,or see how the availability is around that time.... you could head out to a couple of billabongs that are close to darwin, corroboree is onnly 1 hour away,and if the weather starts warming up around then, the barra should be on, evan if you catch nothing out there it is a spot not to be missed.... if you require any more info just let me know
Cheers Brad