What took him there nobody knows. Writing to the Trinity House about the
lighthouse business on the 8th of May 1662, Marvell says:--

"But that which troubles me is that by the interest of some persons
too potent for me to refuse, and who have a great direction and
influence upon my counsels and fortune, I am obliged to go beyond
sea before I have perfected it (_i.e._ the lighthouse business). But
first I do thereby make my Lord Carlisle (who is a member of the
Privy Council and one of them to whom your business is referred)
absolutely yours. And my journey is but into Holland, from whence I
shall weekly correspond as if I were at London with all the rest of
my friends, towards the affecting your business. Then I leave Col.
Gilbey there, whose ability for business and affection to yours is
such that I cannot be wanted though I am missing."

It is plain from this that Lord Carlisle is one of the powerful persons
referred to--but beyond this we cannot go.

Whilst in Holland Marvell wrote both to the Trinity House and to the
corporation on business matters.

In March 1663 Marvell came back in a hurry, some complaints having been
made in Hull about his absence. He begins his first letter after his
return as follows:--

"Being newly arrived in town and full of business, yet I could not
neglect to give you notice that this day (2nd April 1663) I have been
in the House and found my place empty, though it seems, as I now
hear, that some persons would have been so courteous as to have
filled it for me."