Where can I buy a train ticket from Utrecht Central to Hoek van Holland Haven online?

NS no longer operate trains to Hoek van Holland since 1 April 2017.

Since 1 April 2017, the line between Schiedam and Hoek van Holland is operated by RET. For the first several months, this segment will be operated by a bus, but later it will be a kind of urban rail / metro train. As it is operated by RET, not NS, You cannot buy a ticket to Hoek van Holland through NS anymore.

Online information on this appears to be mostly in Dutch, such as through Prorail, through Metropoolregio Rotterdam-Den Haag, or through NS.

As for the ticket alternatives that do exist:

• Anonymous OV-chipkaart; buy once for €7.50, recharge and use as often as you want. If you already have one, this is normally the most economical option for domestic travel.
• Single-use ticket: one for NS Utrecht–Schiedam, online or at ticket machine / office, and one for RET Schiedam–Hoek van Holland.
• If you are aiming to take the ferry (as many travellers to Hoek van Holland Haven will be): Rail&Sail ticket includes train and ferry from anywhere in The Netherlands to Hoek van Holland (see also this question).

The Rail&Sail ticket needs to be bought in advance as Stena Line will need to send you the ticket by mail (for travel on NS and RET). For the others, there is no economic benefit in buying in advance, if at all possible.

Basically you can't, not from the NS. As you noticed, e-tickets can only be paid for using the iDeal system available to Dutch banks' customers, not with (international) credit cards. The only solution available for train journeys in the Netherlands is to book through a third party. Both the Belgium National Railway company and trainline sell tickets between Utrecht and Schiedam Centrum for €12.20.

Buying from the NS at the station would cost you about the same: €11 pay-as-you-go fare to which you have to add a €1 charge for a disposable chip card and €0.5 credit card fee. Buying from a desk is also possible but comes with an extra service charge.

Note that there is no penalty for buying a ticket at the last minute, no seat reservation and all tickets are in principle valid on all trains. Think of the Dutch train network as a glorified public transit network rather than as a long-distance train network like those you might be used to in the UK or elsewhere.

No matter what you do, the bus leg (or the metro when it will be available later this year) will have to be paid for separately. I would recommend buying a regular 1-hour ticket from the driver. If you stay longer in the Netherlands or plan on coming back, buying an anonymous OV-chipkaart might also makes sense but the system is not very visitor-friendly (and deliberately so).

Finally, note that to make things even more confusing the NS website is able to provide public transportation routes to most places in the Netherlands (not only train stations). That doesn't mean NS tickets are valid on bus or trams, you still need to buy a ticket from the relevant local transit company.