IMO, your portfolio should portray both your technical knowledge and design flair. Your designs should stand up to scrutiny by those who have an innate feel for whether the layout is sticking out through the sides or whether the head room wont fit under the foredeck or whether the massive tender fills the space where the engines would be etc etc. In other words, put time into thinking the design through....style is fine, but check there are no howlers lurking in your design that an interviewer will relish pointing out to you in a well rehearsed holier than thou manner.
The usual format is basically profile, deck plans and a 3-D exterior / interior or two...throw in some detail views too if you can.
Underline your designs with some basic Principal Characteristics...major dimensions, capacities, displacement, engine choice / performance expectations and such.
Hand drawn sketches are a distinct bonus....these days computer literacy is a given, so although computerised renderings are effective and dynamic, hand drawings (especially well proportioned '3-D' views with good perspective) are a breath of fresh air to prospective employers and give a great indication of a designers eye. The ability to actually draw free-hand is sadly rare in our industry these days. That said, most companies these days use high-end design software....if you are conversant with say, Rhino, Solidworks etc, put examples in there and make it clear that you are aware of other software and are capable and willing to learn if need be. Can you render ? Add a couple of images if you think they look viable.
Any ideas on 'special features' ? Such as sliding roofs, tender garages, balcony doors, unusual features etc...if so, a couple of sketches with notes and or renderings showing tantalising detail will get them interested very quickly.
As to what designs to put in your portfolio....do you have any favorite builders ? Do you have a leaning toward a specific type ? Perhaps you can target a particular size / type you like and adapt it / them. Portfolios exist to spread your bets, so unless you want to focus on one or two prospective employers, a broad mix of small / large / motor / sail / interior / exterior is a good bet, although intense to prepare.
Using an existing design or two of the size you are interested in is a good start to getting a feel for what a boat of a certain size can fit in it...I don't mean copy, I mean at your stage of learning and experience it cant hurt to get a feel for what established designers do to package up the same old problems and compromises, learn from them and add your individuality to an accepted viable platform.
I suggest starting with a hull of a pre-determined size and type...give the existing / potential competition a look to get a feel for number of cabins, deck spaces, machinery options etc. Get a layout roughed out, with a few check sections for floor widths and headrooms, then style it up.
Most interviewers who may employ newbies / graduates are looking for long term potential, and have been around the block several times...they will spot flair and or technical ability within the simplest of designs as long as they are accurate and attractive.
Good luck