The sea is calmest in the morning and all transport stops running in the late-afternoon, well before dark. During periods of southerly winds and in July and August especially, the swell can be a bit hairy and you are very likely to get wet on the crossing. It is advisable to place laptops, cameras and handphones in waterproof bags for the crossing. You will need to wade through shallow sea when disembarking, so wear either appropriate footwear, or none at all.
From Lombok
The daily Perama (http://www.peramatour.com/) service will take you directly from Senggigi departing 9AM to Gili Trawangan in under two hours for a flat Rp 100,000, plus optional Rp 10,000 for pickup at your lodging in Senggigi. Other operators can swing similar deals for Rp 75,000, but these often involve going through Bangsal.
Another option is to arrange a speedboat charter or pickup with your hotel, or a dive shop on Trawangan. These speedboat services run in and out of the lovely Teluk Nare bay in Lombok, south of Bangsal and thus avoiding the thoroughly unpleasant experience of combating the touts there. A speedboat charter normally takes up to three passengers, crosses in 20-30 min, and will cost about Rp 500,000.
The drive north from Senggigi follows the coast and the road offers panoramic views and glimpses of roadside village life. It is not a lengthy trip and the road has recently been upgraded throughout almost the entire length to either Teluk Nare and Bangsal a little further to the north. It is a quite scenic trip and glimpses of the Gilis can be seen as the road rises over the headlands whilst travelling up the coastline.